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 so–called Bonus Campaign (‘OrcX’) in The Frozen Throne — the one with Rexxar the Beastmaster which plays like a quest–based RPG.
I don't know when this incident began, but it could've been when I was exiting Thunder Ridge and returning to Durotar.
I tried restarting War3.exe, but that didn't change anything.

Both my heroes are at level 8. Also, their numbers are almost the same: at 4400 points required for level 9, Rexxar has 3506 and Rokhan has 3504.

Furthermore, I've since done enough that I would expect my heroes would've gained a level by now. My heroes have slain a few enemies at level 8 and 9, among other things.
If there is something which prevents gains at certain points along the quest progression, then one would expect that using Tome Of Experience would similarly be prevented. Maybe the designers didn't think of that.

What can be done to force the meter to refresh — or whatever else is necessary?