Timing Slot
Walkalong...Remove the slide and flip it upside down. Near the rear, there's a shallow slot that looks like a keyway in the center rail, and in fact was machined with a keyway cutter. That's the area that the disconnect's head pops up into and lets it reset between the trigger stirrup and the sear. If it doesn't move high enough, the disconnector spade can't fill the gap between trigger and sear.
Other issues can cause the disconnect to fail to reset correctly, but ageni's noting that the gun failed to fire when letting the slide move forward slowly makes me suspect that the depth of the slot is the culprit.
FWIW...If the resetting of the disconnect comes with a distinct, audible "click"...the slot probably needs to be deepened a little anyway.