Only a couple of possibilities (IMO)

If the gap between the magnet and the armature is too large, the magnet can't pull it in when it get's hot.
-adjustment depends on the type of brake, but a quick setup method is to tighten the assembly together then back the 3 adjusting screws off~1/2 a turn each.

The second possibility is that you need a new brake. In a nutshell, the heat causes the insulation on the magnet windings to break down, and as they short together the magnet gets weaker and weaker.
-be prepared for the price shock :-)

The newest nacco electric brakes are completeley non-serviceable and non-adjustable. They are in spec or they're replaced... so much for progress.