I bought a stripped Stag Arms lower and a Stag parts kit. When I installed the magazine catch, spring and button I noticed that while pushing the button it would catch and not operate. I took it out a few times and reinstalled but not luck. When I place a magazine inside the button for release works fine but the mag needs to be jammed and slapped really hard to get it to install. The magazine is getting damaged by sharp edges. I smoothed over the edge but still sometimes it sticks and the mag is still not locking in smooth. I have assembled 4 Stag lowers before and never had any trouble with this part.
Make sure your parts are ok. Especially the spring.
Compare these parts to some that are working.
If the mag catch itself is ok you could have a spring problem.
Install the mag catch without the spring.
Does it move smoothly in the normal range of travel? If no then find what is binding.
Use your finger to simulate the spring. Does the mag insert ok?
A few weeks ago someone posted a problem with what appeared to be a receiver that was not machined right.
This is the kind of thing someone who knows how things should look and fit could probably find easily.
You said it is catching. Where is it catching? Half way in?? How are you un-catching it?
Pictures might help. Do the right and left sides of the receiver with the parts installed.