Bike wont shift to highest gear when i sit on it.

I have a full suspension mountain bike. It is a 1x12 speed. I brought it to the shop for a tune up, and the deraileur works perfectly on the bike stand. However, the moment i sit on it, downshifting is good, but upshifting after gear 5 is janky. And I can only shift up to gear 11. As long as I am sitting on the bike, the bike will not shift to gear 12.

The moment i hop off the bike, and lift the bike up, then use my hand to crank the pedals, it shifts to gear 12 perfectly. Is there any fixes to this?