Can't repair structures when attacking enemy stronghold

So infurating! When playing Dawn of War: Dark Crusade as the Space Marines, I can't repair my structures with servitors. The servitors go up to the structure, the repair animation plays but the structure's HP does not go up an inch.

Like I say, it works fine when playing normal maps... just when I go up against a stronghold I can't repair my structures!

Is this a known glitch? Really irritating.