What kind of ammo was being used, new or reloads? I have found that if I have my thumb positioned incorrectly on a couple of my 1911a1's, the slide will lock when firing. Try to load 5 rounds, and please check your thumb position after each shot.
"A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed."
Life Member NRA
Life Member NAHC
Former President of the ECPT (Eifel Combat Pistol Team)