Agreed that a few months does not a crisis make. Patience is the key, but there is a few things you can do.
First, I would do is push a bit harder in terms of actively going there weekly and checking on progress. Not to be an a**hole or anything, but you don't want your project getting bumped for other projects around the shop.
Second, make it clear that no money will be paid until the work is actually performed and stick to it. Payments are rendered when milestones are achieved and not before.
Personally, if it takes a few more months to ensure a solid build, I would have no problem with it. If its that there were other priorities and yours is not done because your priority was not as high, that's another story. They may simply be bad at estimating the tasks or the job was bigger (i.e. have to deal with more issues) than they thought.
Thoughts?
-- Joe