String trimmer line slipping out

I bought a Hyper Tough weed eater from Walmart a couple years ago and it runs great but last year the sting started slipping out without bump feeding it. I can start with about 5 inches of line out, and within 5-7 minutes of use the string will be close to 18 inches long, so I have to wind the string back in but 5 minutes later it's back to 18 inches. I've cleaned and checked the head and spool for damage but it looks good, I've tried different types of string but it makes no difference. Is it possible that the head or spool could be worn out?