I use pmags empty mag will not drop free on its own. Even with fairly vigorous shaking it only sometimes throws the mag. Haven't tried any other mag types yet. Is this a normal thing? Can I do anything to fix it or is it an annoying problem I just have to live with. It's a Stag lower receiver. Thanks for the input

Just a noob myself, but are you sure the mag release is fully releasing the mag?????

Sometimes it just happens.  I've had 2 lowers by the same manufacturer where one had mags drop free and the other had them stick pretty tight.  Sometimes it's just a lower with machining tolerances pushing the tighter end of the spectrum.  

It might also be the magazine; some mags are thicker than others.  I believe the "slimmest" mags are the old metal USGI mags.  Maybe try some of those and see if they drop free.  Hope this helps.

More than likely the issue is with the mags.  Try a metal USGI mag.  I use both and the metal mags consistently fit better and drop easier than poly mags.

I have two stag lowers and Magpul gen3 wont drop free.  Gen 2 magpul and usgi mags no issues

My friends DPMS will not drop free pmags, but does USGI mags.

Had that problem on one before. Problem ended up being the mag release was screwed in too much and was dragging the mag even when button was fully depressed.

As was mentioned, make sure it isn't the magazine catch set too deep.  Even if it is at the "proper" depth, back it off one turn and see what that does.

If you still have the problem, it is probably a slightly tight magazine well, but I would just rub some sandpaper on the magazines until they drop free.  If it is the magazine well, you will see rub marks on the magazines that will give you an idea where they need to be slimmed some.  We are talking about basically breaking the surface of the plastic, so you are not altering the magazine very much at all.