First question, how old is your reel? If it's 20+ years old there was a design change due to problems with the levelwind and you will have to upgrade the whole levelwind system including worm gear, gear shield and bearings...How do you know if it's the old style? If it is a plastic line guide you have the old version and need the updated package...if it's the newer version it will have a metal line guide...
Check to see if the line guide itself is bent and/or is caught on the line guide post (highy unlikely)...
Next check to see if your reel frame is twisted due to the strain from the high line tension...sometimes the frame will twist and throw things "out of whack" causing the levelwind to bind...
Next check your pawl which is located at the bottom of the line guide behind the big screw...take it out and see if it is bent and/or worn...replace it if necessary... most times when the line guide doesn't work is because the pawl is worn...
try these and see if it helps...if not it may be an internal problem with a gear or bearing...
Give me a call if you have any questions, I'll be glad to help if I can...443-848-1074