You likely have the RPM too high. Try lowering the RPM to around 1000-1200 rpm. You want them just high enough to keep from stalling the engine when engaging the PTO.
I am not familiar with how your tractors PTO engages but you need to try to slowly engage the PTO lever if possible. Some levers work like a clutch and can be slowly engaged to allow some initial slippage. MY LS is engaged with a push/pull electric switch but by fully depressing the clutch (it is gear model) I can slip the clutch a bit to slowly start the PTO without damage to a shear pin. My brother in laws NH 2310 (HST) is able to slip the PTO clutch by slowly engaging the PTO lever.
Regardless of how your tractor engages the PTO, the key is low RPM to prevent shearing the pins. Also you may want to go with a harder shear bolt if it continues to shear. I put grade 5 pins in my brother in laws plus make sure the pin is a tight/exact fit to the shear pin holes.