Before operating Bosman Supplies, I worked at 3M for a few years. My job was to recommend adhesives to the defense markets.
I would receive calls multiple times a week saying that my glue had quit working, or that the sample I had sent didn't work in their application.
We found that about 80% of the time it was not the glue. It was usually a change in employees, process, substrate, or lack of surface prep. Of that 80%, most of the failures were from a change in employees or lack of surface preparation.
Up to 30% of your surface is generally covered in dust and debris. I always recommend cleaning with a 50/50 mixture of rubbing alcohol and water. Allow the alcohol to flash off, then apply your adhesive. This is especially important with plastics. You may only be getting 7 psi peel strength with your adhesive, without surface prep this can drop to 4 psi peel strength. This is a lot of strength in smaller numbers.
The next time you have a failure, make sure to check through the list above. If it is still failing there is a good chance I was wrong. I am always willing to stop by and do some original testing with you to make sure we come up with the right process and product from the start.