In Avengers: Endgame, one plan to undo Thanos' snap is to travel back in time using quantum effects to gather the Infinity Stones again from the past.
A repeatedly emphasized limitation is that they only have enough "Pym particles" for one roundtrip per person.
Why don't they simply start by sending someone back to gather more from a lab, as Captain America and Iron Man actually do later, in an improvised attempt to save the plan?