after analyzing your model:
goto display menu\Show tables
analysis results -> building output -> table: center mass rigidity
it shows story, diaphragm, Xc, Yc , mass X , mass Y
I think you have chosen inappropriate size for your striker. small size of striker and momentum conservation law cause your striker goes back.
There is two solutions for your problem:
1-increase the size of striker by modeling extended setup.
2- increase mass of striker by mass scaling or...