Had the same issue with my new p10s, I found out was was because I was pulling the slide back too far. Also make sure the trigger is in the reward position before trying to takedown.
If you put downward pressure on the takedown bar and start to pull the slide back you will feel when the takedown bar moves, at that point you can move the slide forward and off.
If you hear the sear reset on the striker and you are pulling back for takedown you have gone too far.