Unit unable to construct buildings?

I've got a unit in my map which is supposed to construct defensive buildings, such as sentry guns, barricades and stuff like that, however, whenever I try to build a building, he won't build it, I can still place the "blueprint" and the building will spawn with 5% hp (or whatever amount buildings start with) but then I can't get him to finish constructing the building, what have I done wrong?

You need the ability "repair" on the unit. (The unit that builds, I.E. Peasent)

I thought of that myself as well, but even when I added the repair ability, it didn't work :(

Are you sure the repair ability is repair (human)? If it's the default repair ability that the peasant has, then check the other fields in the buildings and see if there's anything out of place (repair time, classification, etc. etc., not that I think these will solve the problem.)