You did take the safety off, didn't you?
The only thing I can think of, is that it catches on the holdopen latch. That is an L-shaped piece of metal with a small leaf spring. Normally it is down, but the magazine button will push it upwards when the magazine is empty. If this pistol was stored for a long time with the magazine inside, chances are that the holdopen is stuck in the upward position.
You can reach it through the magazine opening.