1911 Slide not locking back on last shot

What could be the cause of the RO 1911 slide not locking back on the last shot?
It happens with all 4 mags. I have an UD Matchdot II mounted on the slide, 10 lb recoil spring. The ammo is 160gr SWC with 4.4 gr Bullseye powder behind.
Thanks.

Does it lock back on just an empty mag when hand cycled?

And, try a 12# recoil spring, maybe cycling too fast

Was it locking back previously and this is a new issue?

Is this a new load you are trying?

Have you recently switched recoil springs?

My ro was doing same when I switched to the 160. I was running 4.2 of wst raised it and switched to lighter main spring.Thats what worked for me.

It does lock on an empty mag. I ran close to 8500 rounds using 160 gr SWC and find it be extremely accurate. It is a new issue, because it used to lock every time, until I run about 250-300 rounds through it. I find that when it gets dirty, the slide slows down and doesn’t quite go into battery. Now I clean it after 200 rounds and it runs consistently, but slide stops locking after about 50 rounds.
When the gun is clean it locks. After running about 50 or so rounds it stops locking.

What lube are you using on the slide and bushing?  What crimp on the load?  Increasing the crimp to the .463-.465 range will let the powder burn more fully and therefor be cleaner.  Generally see an increase in accuracy as well.

I use MilItec lube on the slide and bushing.
The crimp is .469.
Thanks for the suggestions.

It's likely dirty in there. It doesn't take much.
Experienced the same problem with my Volquartsen not long ago.
Stripped it, cleaned it, Problem gone.