I had Windows 10 on my laptop and I've installed Windows 7 on different partition. At the beginning everything was fine, but after 3 weeks Windows Boot Loader started to not showing up, randomly. Sometimes it shows up, but most of the time it doesn't and Windows 10 is loaded immediately. Even on BIOS "boot from" options I can select to boot from "Windows Boot Manager" but then also selection screen doesn't appear and Win10 is loaded instantly.
In msconfig on both Win10 and Win7 there are 2 entries and Win7 is set as default, the counter is set to 30 sec.
I can't see any pattern, except I found out one way to make Windows Boot Loader appear in 100% of times: when I force shutdown when Win10 is loading - the next time I turn on the computer Windows Boot Manager will always show up . But it's not a solution.