I have only ever used a string loop and have found that you can get loops in different thicknesses. I think it might just be what brand you buy. I had the same problem once and changed the string loop for a different brand and she was all sweet.
I now only missfire when the string loop is wearing out and fraying cos it gets thinner and I try to change it before I loss and arrow[:@].
Only lost one arrow through a miss fire in 5 years
Cheers,
Stalkin Steve