Here are two recent examples of union members rejecting a tentative agreement and getting a significantly better contract as a result.
Cleveland Teachers union 2016-2017: Cleveland teachers rejected the first tentative agreement brought to them and approved the second one, thereby fighting off most of Republican backed merit pay plan. They won a better contract.
Fiat Chrysler 2015: Auto workers succeeded in rejecting a multiple tier system where new hires get less than older employees. They did this by rejecting the first TA, and then approving a second better contract.