...or you didn't purify the freebase before gassing, so that the impurities react with the acid forming a sticky goo, which sticks to your product.
This symptom is often (incorrectly) referred to as "overgassing", as it tends to manifest itself towards the end of the gassing procedure. The real problem was there all the time though, and should have been remedied by a/b washings and preferably distillation of the freebase before the gassing.