Sounds like the string is slipping through the gripper on your stringer. It could be when you switch to a 17 gauge string your gripper is not closing far enough causing the string to slip. Could be dirt in the groves of the gripper which is hard to get out without taking the gripper apart. I would not recommend taking the gripper apart to clean it but I doubt you can buy a new one. My guess is there is too much gunk build up on the gripper plates or they are just worn out.
If you decide to take it apart be real careful there are two springs and 28 ball bearings in there to get lost. Not a lot of fun putting it back together again either.
I would like to apologize to anyone I have not offended. Please be patient I will get to you shortly.