The correct way to disassemble it is to use the sear disconnect lever in the mag well. If you read the manual, that's what the second purpose of the tool inside the grip. The first purpose being holding the backstrap.
Mine (M&P Pro 9mm) doesn't spring back unless it's not all the way at the 6 o'clock position, like h20too said.
Unless there was some sort of mid-production change that I'm unaware of, it may just need a little "breaking in."
Korey - Shamrock Armory
Glaine ár gcroí, Neart ár ngéag, Beart de réir ár mbriathar.
Purity of heart, strength of arms, and truth in words.