In *Avengers: Endgame*, Ant-man has the idea to pull off a  "time heist" in order to get back the 6 Infinity stones and reverse the Snap. As stated in the movie, there are no "second chances because they only have enough Pym particles for one time trip. 

Why couldn't they send a couple of people back to find the time stone (which *was* the first -and easiest- one to be found), and once they have the time stone they can travel through time without needing the Pym particles?

**Edit:** After they get the time stone they can then retrieve the other 5 without needing the Pym particles.