This could be one of two things or even both. First thing is take the base plate off and make sure the plate on the bottom of the Mag spring is attached correctly in the correct direction. Trust me that one thing has bitten me square on the butt more than once. The way to tell is the bent piece that hooks onto the spring is sorta kinda round so that part face the front of the mag, and the back that has just that little bend/flap thing in flat so it faces the back of the mag. Yes I am sorry this looks like I am typing this to someone I think is stupid, but I swear I in no way think you are stupid or lacking in any regard. Just keep in mind we do have the occasional international member that may need the extra description I put in to get what I am saying because of things not allways translating correctly.
The next thing once that is done and it's correct. Push down on it just like allways and slide the base plate back over the bottom of the mag. This is the sort of sneaky part that can bite you. When you slide the base plate over the plate with the knob facing up (only way it can be actually) do so slowly and listen for a very slight click, if you hear it that means the knob has seated into the hole in the middle of the base plate. See if the plate still slides off. If so then take a small something and stick it in the hole and see if you can move the locking plate until it does pop up into the hole. If that doesn't work you either have a really dead spring. (not to likely, or the locking plate on the spring is bent down in the middle, Just a hair is all it would take to make the latch/knob fit up into the hole. it also goes for the plate itself. Make sure it's dead flat also
