I had the same problem with some thinner gauge strings (like 18g kevlar) slipping through the gripper no matter how much I adjusted the screw.
Solution: I put a plastic gift card in the gripper with the string. No damage to string and always works.
USPTA P1, Gamma 2pt 6004 -Wise
Head TGK 237.2, TF NRG2 16g 53LBS or IG MP, NRG2 17g 51LBS