2015 i12s seat straps

I have the 2015 i12s and have to constantly adjust the seat back straps as they do not stay secured in one position but slip until they are loose and I am almost laying down. Has anyone else had this problem, and how have you resolved the slippage? I have tried large metal clamps (black, for clamping papers together) with only slight success. Sure would be nice to retrofit the Vantage seat but the 2015s are narrower.

I bought my wife a 2016 i9s and the seat is very comfortable. But, being it is wider and has more resistance, I could not keep up to my nephew who was in my 2015 i12 with even a faster stroke in the i9.