What to do when an inspector misses stuff?

I would talk with the realtor for advice. Hopefully he/she isn't avoiding you now that the deal is closed. If the previous owner knew of the problem but didn't disclose it, there may be some sort of recourse for you, especially if you can find evidence that the area was previously patched. I'm sure you'll eventually need a lawyer so you'll have to consider if it is really worth the trouble.