without photos or any other information it is difficult to say, but it sounds like it may be a moisture problem when it was sealed, you need to find out what type of sealant was used, get hold of some thinners and spray onto the affected areas.
Rub hard with a stiff brush to release the trapped moisture and let it dry again. This sometimes works but not always.
Like I said, without more information it is impossible to be certain of the problem.