Don't feel bad, most of them rarely work well after the first spool of string or so. I have a Stihl straight shaft with an after market head and you insert short lengths of string easily into clamp jaw lined holes you can generally do a whole yard and lots of trees before you need to replace them.

This is an example of the kind of head I am talking about. I have used a similar one for years. Little or no problems.

Here is a link that might be useful: Look here

My only thoughts??? you maybe using too large a string? or when you winding the string it is coiling up and lapping over and knoting. Your spring may not be letting the spoon release (compressing enough to release it) or you may not have enough RPM when bumping to alow centrifugal force to turn the spool and advance the string. If should work beings you can manually advance it. I know SHOULD is a BIG word!!!!!