The Apache License 2.0 says, in relevant part:
- Redistribution. You may reproduce and distribute copies of the Work or Derivative Works thereof in any medium, with or without modifications, and in Source or Object form, provided that You meet the following conditions:
...
4.c. You must retain, in the Source form of any Derivative Works that You distribute, all copyright, patent, trademark, and attribution notices from the Source form of the Work, excluding those notices that do not pertain to any part of the Derivative Works; and
4.d. If the Work includes a "NOTICE" text file as part of its distribution, then any Derivative Works that You distribute must include a readable copy of the attribution notices contained within such NOTICE file, excluding those notices that do not pertain to any part of the Derivative Works, in at least one of the following places: within a NOTICE text file distributed as part of the Derivative Works; within the Source form or documentation, if provided along with the Derivative Works; or, within a display generated by the Derivative Works, if and wherever such third-party notices normally appear.
So if whoever is reusing your software is distributing the source code but without your name and copyright notice, it would seem that they are violating the license.
You mention a README file, but do not mention the NOTICE file which is listed in the Apache license. If you included a NOTICE file, and your software, or a modified version based on your software, is being distributed in object code form (only) without either a NOTICE file listing you, or a visible display by the program listing you, it would seem that they are violating the license.
The license specifically mentions a NOTICE file. It is not clear that attributions or restrictions in a README file will have the same effect. I would suggest that you add a NOTICE file to your distribution (if you did not already have one), and include one in any future distributions under this license.
If someone distributes a modified version of your software in object code format only, and you did not include a NOTICE file, it is not clear to me that the creator of the modified version must include any credit to you. However, the modified files must include a statement that they were modified from the original, and indicate who did so. Absence of that statement would also seem to be a violation of the license.
If it seems that the license has been violated, AND if you know or can find out who is distributing the modified version, you can send a letter (or email or other communication) indicating that the modified software is derived from your work, and should carry attribution to you. You could request or demand that they stop distributing copies that do not carry proper attribution. You may want to ask a lawyer to draft or review any such letter.
If the modified software is posted on a site subject to US law, you could send a DMCA takedown notice about the issue. This is fairly simple, and should not require a lawyer. However, if the other party files a counter-notice, the site could put the software back up if, after 10 days, you haven't filed a copyright infringement suit. Some sites restore challenged content quickly if a counter notice is filed, some just ignore counter-notices.
Ultimately, if the person does not restore proper credit voluntarily, and if a takedown is not available, or doesn't get the results you want, you would need to file a copyright infringement suit. To do this you would want to be sure that you have a good case. You would need to consider upfront costs and requirements, and possible damages should you win, to decide if it makes sense to file suit. Often it does not make economic sense. I would urge you to consult with a lawyer experienced in copyright issues before making a decision to file suit. Such a lawyer could help you estimate your chances of success, and the probable award if you win the suit. (Note that if all you are seeking is attribution, available damages may be limited, unless you can prove that lack of attribution harms you in a specific, economic way.) Such a lawyer could advise on needed pre-requisites (if you are in the US, you may need to register the work before filing suit). Such a lawyer could advise on what evidence would be needed to win, and how good or bad any evidence you have is. I cannot advise on any of that.
