I am in a co op game with my partner and she has more building spots to complete than I do on the same building.
For instance, we are building the tool maker and she has about 10 spots to click and build, I have 5 on the other side, however, when I click and the animation starts it just goes on forever (literally) and nothing gets built.
This has happened on quite a few of the tier 2 buildings so I think it may be an issue with tier 2.