I notice at times that my experience meters aren't showing gains when either

• creeps are slain
• a Tome Of Experience is used

The numbers shown when the mouse cursor is over the bar show no increase; the blue bar filling from the left is not displayed at all.

Stuff like this has happened during normal campaign or multiplayer games for both warcraft-3 and warcraft-3-frozen-throne; the incident which prompted this question occurred with the Rexxar ‘OrcX’ campaign in The Frozen Throne.
I don't know when this incident began, but it could've been when I was exiting Thunder Ridge and returning to Durotar.

Furthermore, I've since done enough that I would expect my heroes would've gained a level by now.
What can be done to force the meter to refresh — or whatever else is necessary?