Could be that the logistic side of the IRGC is not very strong and with a lot of leadership loss and communication breakdown, those that have the stockpiles are not releasing it or hoarding. Or they sold much of their stockpiles on the blackmarket years ago.
It could also be that the Americans are waging a type of war that the Iranians simply can't fight back against...
How does the IRGC fight back against 3 carrier strike groups + hundreds of ground based fighters?
Space based assets? (Including some sort of space based DEW I saw video of?)
High flying bombers like the B-2 and B-52, especially in an environment totally saturated with jamming signals?
The ammo issue may be irrelevant at this point because there isn't anything for them to really shoot at. Not with rifles anyway, or even AA batteries.
...
And then there is undoubtedly armed rebel groups or some sort of armed underground movement that will be assisted and directed by US SOF & intelligence agencies to do some damage from the inside...
Hopefully a lot of the rank & file just go home. Be good parents, and good neighbours. Go home & take care of people.
This fight is so far beyond what an average Iranian soldier can possibly fight back against...it isn't a battle as much as it is a systematic dismantling of the regime and it's military assets.
Caveman vs Star Trek levels of tech overmatch here
EDIT - How do they fight back against that? Asymmetrically, I assume.
Cyber attacks. Sleeper cell attacks. Attacks on domestic infrastructure primarily in the US & Israel. Etc