Funny, I had the exact opposite happen to me when I installed the MagPul PRS buttstock. But then, my AR-15 is a Daniel Defense DDM4.

Perhaps the difference was that I also got a different spring for the buttstock. It was longer and came with a hard plastic spacer.

The rifle cycled fine prior to installing the new buttstock but, afterwards, it wouldn't fire. It felt like it was short cycling but, it looked like it was going through a full cycle.

I looked through the box and found the spacer and added it.

After that, my DDM4 fired fine.

Now that I've got that working, I'm thinking of adding an adjustable gas block to dial down the gas pressure to improve accuracy.

These things are definitely temperamental, aren't they. I try to keep it down to one change at a time then take it out to the range to test before I move onto another change.

I wonder why we had opposite experiences?