Hmmm, two ideas:
1) Is the unit's invitory full?
2) Does your shop have the ability to choose different units? I forget what its called. Look at a premade shop, you should see the ability. In-game it looks like two arrows going around in a circle.
Other than that, I don't know what the problem is.
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.