There's a good possibility that the problem isn't "short stroking", as in the bolt isn't "coming back far enough".
In most semi-auto firearms, it's almost impossible for the bolt NOT to cock the hammer due to the design. In most autos, the hammer cocks before the bolt opens enough to eject a round.
In many cases, what's actually happening is a problem in the trigger group is causing the hammer to fail to STAY cocked.
In other words, the hammer is being pushed back, but something is causing the hammer to not be held by the disconnector, and the hammer is riding the bolt back down.
This could be a worn, misassembled, or fouled trigger group.
My first move would be to check the trigger group, and/or give IT a good cleaning.
Look for grit, fouling, brass and burned powder particles, etc.
Test the trigger assembly to insure the sear/disconnector is functioning properly.