Had a pretty pleasant day at the range - my first trip in over 6 months.

Fired my Rhodesian FAL for the first time - VERY nice and accurate.

Fired my new 9mm upper with FIRSH handguards, front grip, EOTech 511 holosight and VLTOR Clubfoot stock.  VERY VERY nice, accurate - I used a 10 round modded Oly mag, and then a magblock with 30 round Sten mag - functioned flawlessly.  

Now, the problem:

I have a DPMS .22LR upper that I have never fired before today.  I had it mounted on an Eagle lower with 2-stage NM trigger group.  So, first it wasn't really wanting to chamber rounds - seemed sort of weak.  But I figured that maybe it was just the ammo.  Well, it seemed that every other round it was not firing.  It appeared that the firing pin was just not striking the .22 rounds hard enough to set them off.  

So I switched the lowers out, and I still had the same problem.

So, I don't think it is the hammer spring.  Plus, both lowers worked fine with the 9mm upper.

I bought the .22 upper used.  Is it possible that the firing pin is too short?  Worn down?  Any other things you all can think of?  When it did shoot, most of the time it ejected and loaded the next round.  Now, it did have a preference for ammo - it did NOT like the SSS Aguila, and didn't load HP very well, but it seemed to like Remington and Winchester.

Anyone have any thoughts or ideas as to what could be wrong?