Helping a guy remotely on why his bolt release isn't working.  Says his release is missing the mag follower.  Any ideas?  Says he only has 1 mag to try.

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Seems to be an issue with the pocket depth on this lower. Out of tolerance. Not allowing the bolt release its full travel.

Trying different magazines would be a good idea. The hold open looks normal to me but it looks like the top of the magazine/follower is too far forward in the mag well. Maybe it's the magazine or maybe it's the lower causing it? You should be able to wobble a magazine forward and backward in the mag well without the follower slipping past the catch, assuming the magazine and follower are in spec.
You can mod a 9mm catch to get more arm length, if that's what ends up being needed. You should be able to find threads here about how to mod a 9mm bolt catch.

The guy ended up grinding up the bolt catch to remove material. Seemed to have worked. The part now has full travel and is working.

It’s an out of spec 80% lower. You know, the ones on sale cheap that are only cosmetic blemishes. Don’t worry you’re not the only one that’s got one. Don’t know till it’s too late like everyone else. The mag well is cut too far forward. Compare to mill spec drawings and you will see it’s .030-.060 off which is enough to just miss the mag follower or ride off of it when it raises. I had two last year same problem. Couldn’t tell where they were from as they got mixed up with other manufacturers. There’s a bunch of them out there. If the problem comes back just do the 9mm bolt catch mod. It’s easy and you don’t have to worry about it again.

It doesn't have to be an 80% to have this problem.  I have a Spikes that had the same problem. As already said several times, the 9mm bolt catch solves it.  Just trim it down so that it'll catch the follower.  Just remember when doing this, if you take off too little, you can always take off more.  Take off too much...  Don't get in a hurry and be prepared to do a few fits.  Instead of having to drive in the roll pin and remove it multiple times, use a take down detent (or other small easy to remove slightly undersized nail...) to hold the bolt release in place for testing.