****IMPORTANT CLEANING NOTE FOR ALL****

[WilliamG]

^ ^ ^ I'm sure most people have done that (I know I have).

The problem is the takedown lever is VERY sensitive to vibration/movement, so it can actually shift into that diagonal position as you slide the slide off/on. The key is just to make sure it's kept vertical as you slide the slide back on. Easiest way to do that is put your finger on the slide release as you reassemble, to make sure it doesn't shift.

 

[ET.]

My M&P does not have the mag disconnect & I routinely field strip mine by simply pulling the trigger when I return the slide to battery with the breakdown lever pulled down as well. As soon as I pull the trigger (with the breakdown lever pulled down) the slide releases. When I reassemble the gun I pull the trigger as I reinstall the slide & I never touch the sear lever. I assume that the mag disconnect changes things. I'll leave that up to the guys who have that wonderful innovation. Good luck...

One thing I've noticed though (same as an above post). The slide, when reassembling, can appear to be locked when it is put back on the gun if the breakdown lever moves back up a little. Sometimes I have to push the breakdown lever back down before I can move the slide in either direction. This might be what happened to the OP. If that's not it then I have no clue. Just a thought...