Most of the time I've had that happen it was junk in the rewinder. It slows down the return and allows the rope to loop a little.
Fun getting that spring back in there isn't it?
You may have a weak spring, you can usually get them and a new spool from the Oregon catalog.
I really like my old Poulan, trimming saw. It is an antique now and just keeps on keeping on.
Fresh fuel and occasional spark plug, will generally keep it starting on the 3rd or 4th pull. The first time it pops, move it to half choke, so you don't flood it.