For issues raised here on Meta.SE, the advice in this answer about editing to clarify or answering to strengthen the argument is sound. Especially for feature requests, sometimes the actual request (in the question) is poorly-argued but a strong answer can help. Requests or arguments based on actual data seem to particularly impress the team, so brush up on your SEDE skills. (For general advice on writing good feature requests, see this answer from Shog9 on Meta.SO.)

Bug reports and feature requests can also be made on per-site metas. That's ok too; I sometimes see comments there saying "you need to ask this on Meta.SE", but it's not strictly true -- the team is supposed to monitor per-site metas too. That said, there are times when it makes sense to bring it here, which I'll get to in a moment.

For an issue raised on a per-site meta, particularly for a bug report or localized feature, if you're not getting any response from SE and the meta request has decent support from the community, then you can ask a moderator to escalate. That part about support is important; get your community to show up and vote on meta. How much voting you need depends on the magnitude of the request. When moderators ask CMs to do things like burninate tags or add a new migration target or increase the number of custom close reasons, the CMs pretty much always respond with a request for a meta link. As a moderator I've been able to get localized things like this addressed by escalating a meta request to the CMs.

Don't bring things here to Meta.SE that only concern one site, like the examples I just gave; people here will just close it as "belongs on a per-site meta". However, sometimes a request has broader implications -- either other sites might be interested in the fix too, or making the change would somehow affect other sites and not just your site. In those cases, you can get more input (both positive and negative) by bringing it here. When you do:

• Do the stuff in that other answer I linked to present the problem clearly and persuasively.

• Cite past discussions. If a feature request here on Meta.SE arose out of a problem already discussed on your site's meta, link to that and summarize the discussion.

• Explain why this has broader application than just your site -- why are you bringing this to Meta.SE other than that you didn't get a response from SE on your own meta?

• Show some understanding of the impact and effort. Is this a big change but you think it's really important? Show us your analysis. Do you think it's a small, harmless change? Why? (If you're wrong, you'll learn something that way.) What alternatives (for FRs) or workarounds (for bugs) did you try first, and why didn't they work?

None of this guarantees action, of course; you can get more attention for old issues and add strong arguments to support them, but in the end SE will decide.

For issues raised here on Meta.SE, the advice in Sonic's answer about editing to clarify or answering to strengthen the argument is sound. Especially for feature requests, sometimes the actual request (in the question) is poorly-argued but a strong answer can help. Requests or arguments based on actual data seem to particularly impress the team, so brush up on your SEDE skills. (For general advice on writing good feature requests, see this answer from Shog9 on Meta.SO.)

Bug reports and feature requests can also be made on per-site metas. That's OK too; while you may sometimes see comments there saying "you need to ask this on Meta.SE", that's not strictly true: the team is supposed to monitor per-site metas too. That said, there are times when it makes sense to bring it up here, which I'll get to in a moment.

For an issue raised on a per-site meta, particularly for a bug report or site-specific feature, if you're not getting any response from SE and the meta request has decent support from the community, then you can ask a local moderator to escalate. That part about support is important; get your community to show up and vote on meta. How much voting you need depends on the magnitude of the request. When moderators ask employees to do things like burninate tags, add a new migration target, or increase the number of custom close reasons, the CMs pretty much always respond with a request for a meta link. As a moderator, I've been able to get site-specific things like these addressed by escalating a per-site meta request to the CMs.

Don't bring things here to Meta.SE that only concern one site, like the examples I just gave; people here will just close it as "only applicable to one specific site". However, sometimes, a request has broader implications: either other sites might be interested in the fix too, or making the change would somehow affect other sites and not just your site. In those cases, you can get more input (both positive and negative) by bringing it here. When you do:

• Do the stuff in Sonic's answer to present the problem clearly and persuasively.

• Cite past discussions. If a request here on Meta.SE arose out of a problem already discussed on your site's meta, link to that and summarize the discussion.

• Explain why this has broader application than just your site -- why are you bringing this to Meta.SE other than that you didn't get a response from SE on your own meta?

• Show some understanding of the impact and effort. Is this a big change but you think it's really important? Show us your analysis. Do you think it's a small, harmless change? Why? (If you're wrong, you'll learn something that way.) What alternatives (for FRs) or workarounds (for bugs) did you try first, and why didn't they work?

None of this guarantees action, of course; you can get more attention for old issues and add strong arguments to support them, but in the end SE will decide.