Join Date: Feb 2017
Location: San Carlos, Colliefornia
I have the kit on my 15 GTS (born on date 12/2014), I have no issues at all. In the beginning I had no upshifts but that was determined to be I wasnt retracting the shifter lever far enough to move to the next gear. But you said it is not a shoe thing, I would call the manufacturer of the STK. I called him after install and he straightened my issue out.
I would look back and make sure the spline timing is correct, basically take it all apart again and start over from scratch. If youre convinced everything is correct, I would put the OEM back on and see if its still there. If so, its not the shifter. Good Luck