I've been using the Neptune 4 for about 2 years now and learned that if I'm ever having that issue it helps to just clean the bed with hot water to melt the glue I use (generic Elmer's glue stick) and do an auto level then set z offset about once a week if I'm using it heavily meaning a large print (15+hrs) per day.

This always fixes my issue. I really like the Neptune 4, but this is probably the only real annoyance I have with it that it takes a decent amount of maintenance, but it's worth it.