Hi, welcome to the GLFW forums!
It’s possible that you may be hitting a hardware limitation on keyboards for how many particular keys can be registered simultaneously, this is called rollover: https://en.wikipedia.org/wiki/Rollover_(key)
If you want to check that your program is using GLFW correctly you could first try the GLFW test program events.c. If this works but your program doesn’t there may be an error in how you’re using GLFW.
If you’re still having issues then let me know, along with what operating system your using (and which window manager if you’re using Linux).