Hammerli lock back failure and dry firing question

I have been really pleased with my Hammerli International: it feeds anything I offer it, haven't had a failure
to feed or fire in the about 500 rds I have put thru it.
Today  it stopped locking back at the end of a magazine, more than 50% of the time.  I will clean it for sure,
and that leads to yet another question:
Dry firing has been a serious program for me since I got back into BE 3 years ago.  With centerfire, no issue.  But 22 is a concern- I am queasy about 
the drywall anchors not living up to their rep, and the dummies I have won't feed in ANY automatic I have, and I
have most of them. If there are brands that will feed, I would greatly appreciate hearing about them.
About the first question, is there a mechanical fix, like higher register of the magazine? I am using factory mags.

Cleaning and some oil may solve the lock back issue.

A strip off of a magnetic business card stuck to the breech face is how I dry fire my Hammerli.

I like these chamber plugs from Red Feather

https://www.redfeatheroutfitters.com/dry-fire-plugs-22.html

They wear out but last around 100 dry shots

I think the drywall plugs are not good being soft enough to be eaten through after even one dry fire.  Same for the Hammerli type flags after a few fires. 
I will stick to the magnetic card.  It lasts a long time

On your lock back issue, check the slide stop for looseness. Not uncommon for them  to work loose

And look for grease/grit in the magazine follower.

Good luck