It is possible that multiple lines have not had the ringing configured correctly on the line for that device. Even though it's a shared line across multiple phones, you have to configure each and every line for the ringing state. IE Ring one, then blink, etc. If configure it the shared line one device, it does not replicate to the other devices.
Possible do a BAT change on those lines to set to ring, reset the phone. Then go back and set to silient and reset the phones. Maybe just a toggle between the two settings will fix it.
Also, check your phone firmware. There are have been some goofy firmwares lately and I have been backing down a couple version to make a few things work correctly. (or if it did work before, but now it does not.... probably firmware on the phone)