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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?
The plan is to retrieve the stones; they do have enough Pym Particles to achieve this, and they want to minimise any risk to themselves or creating additional branches of reality, so adding in an extra trip which may be unnecessary would be a chance they didn't want to take.
Of course, the out of universe answer is that it adds drama to the scenes - that they just have this one shot to try and solve the problem, and then when the trip to get the Tesseract fails, they improvise the plan to go get more particles.
Based on the logic you suggest, they could just decide to go back and fetch Hank Pym into the future so that he could make as many particles as required to complete the mission successfully.
Circumstances.
When the team was planning their focus was to locate times and placed where they could get to the stone for sure. They identify that in 2012, in New York, there are three stones. So they configure their teams based on this information.
Getting the Tesseract is ruined by a stairs-haiting Hulk and they think everything is lost. It's only when pushed in to a circumstances where they cannot go back empty handed that is occurs to Tony that the Tesseract and Pym particles were together in 1970.
The original plan has them wanting to make as few jumps as possible to ensure that they don't create Stray Timelines (which they eventually do - Loki disappearing with the Tesseract for example).
