The gun shoots accurately and powerful again. I described what I thought the problem may be, so I relieved the pinched metal flange and now the piston goes all the way through and the trigger latches. Took two minutes of work.
All over the Internet I've read about "my gun won't latch", but I haven't found the first remedy. This must be the weakest part of the Walther air rifles.
My info in the meantime is, was the gun worked on recently? No, then it's the small spring in the trigger unit. Yes, then it's the Walther Safety Device, the piston has been gouged somewhat, or the new seal is too thin or too thick.
Period. All are readily recognizable, and with the exception of the trigger unit, easily repaired.