Possibilities: dirt in the half-cock notch, too much (or too thick) grease on the fly.
The fly does not "engage" the half cock notch. It allows the sear to slide over the half cock notch upon firing, but when cocking the lock, the fly should "flip" out to the way, allowing the sear to engage the half cock notch.
Is this a new lock, or has it been working up until the fly replacement? Does the problem occur whether the lock is in the gun or out?
Take the fly out and check the half cock and full cock functions. Reassemble with a little lube oil or light grease.
-Ron