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.