I agree with Paul- it is probably because the affected area isn't clean enough.

Thing is, there really is no way to get everything properly clean while the unit is on your head so all you can really do is get it as clean as possible and hope for the best. Until the base is properly shampooed there are going to be some oils and glue residue trapped in there that can compromise the bond.

In other words, it is difficult to make touch-ups last as long as a proper bond.