Yes, ammo that they also do not have.
Either they have to expend a whole Iskander missile to immediately destroy the HIMARs as soon as it is spotted and stationary.
Or attack it from the air of which they do not have many precision bombs anyways and are barely running sorties.
UAV based munitions are also not commonly used, we see some videos come out but they are just a few for a war scale this big, their should be several 100s of drone strikes and air strikes each week (at minimum), but we are not seeing that. If that was the case, without big inventory stocks would run out fast as well.
That whole Iran-UAV news may also be completely wrong. It would help them alot if it is true. Russia is just going to have to absorb this problem, which I will assume will only get worse especially if certain other missiles start to be transfered, and woe to Russia if this F-16 training is complete and actually followed through by the US (with HARM loadout), and they also train them on PAC-2/3 systems.