Shooting it one handed and it works fine? It might indeed, be a Magazine issue, if I'm reading your post correctly. If its wiggling or rubbing the slide during cycling. When I bought mags for my Witness Match, I bought some Mec Gars. May or may not make a difference, but worth considering.
Have you tried other ammo types/brands? It might not matter, but it's an easy check. If your shooting UMC or Wally world Federal, they are typically OK, but your gun may need something a bit stouter to aid in function.
How many total rounds are through it? If it's still got that new feeling, you might want to think about some more break in before getting too worried.
Recoil spring. IMHO, the witness pistols are a bit undersprung. Stepping up one notch on the scale might help.
I make no claim as an expert, just putting out some ideas.
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