The CMMG banshee is a nice option, uses Glock mags. Mine doesn’t like 9mm WWB ammo thou but seems to run everything else. I’ve also run Magpul mags with no problems.
A friend of mine who works for a gun shop / range says that he sees a lot of people have issues with WWB in many gun though.
If you read the instructions for the endomag you will find that they have a brake in period by just cycling the bolt over the mag sever times by hand. Yours is more then likely f#@%. Also they can be very finicky with certain bolts. 100% reliability is the mag inserts and use glock mags.