I've been with Enercon since 1998, and have spent a lot of time travelling and doing field service. One of the things initially surprised with was the amopunt of service calls that related to application issues. I quickly learned that my "Mr. Fix-It" hat had to be taken off and replaced with a a detective's hat.
When I encounter seal problems, I ask a series of questions. I always start by asking if the sealer is aligned properly for the bottle being sealed. It's amazing how many customers don't verify this regularly. I usually recommend checking the alignment at the start of every shift, and after a product change. A small amount of time on the front end can save you many headaches!
The next big question is: How are your application torques? Again, "I don't know" is a far too common answer. Inconsistent torques are a common source of seal problems. The pressure on the seal created by the torque is one of the critical components for a good seal to happen. (Pressure-Heat-Time)
Finally: Has anything changed in your process? This can be the most frustrating to figure try and find. A list of possibilities that I have seen includes: Line speed change, Ouput Power change, Cap change, Liner change. Once you start digging into things, you may be surprised with what you find out.
If you'd like some more information this topic, check out, "Achieving the Perfect Seal."