Apparently Javelin manufacturer can't replenish US inventory quick enough and US has send a considerable fraction of its own Javelins.
Russia has now captured intact Javelins that they don't know what to do with. The high rate of Western equipment losses is a given. A huge reason why the West doesn't send any serious military equipment is because they know the Russians may capture them for evaluation, study and determine how best to counter these weapons. The Americans even ripped out an important component in the M777s sent.
Reading more on the DFCS on the M777, probably it got ripped out because it enables encrypted communication between command post and gun to more precisely position the gun. I don't know anything about cryptography, but I imagine they don't want any software at all in the hands of Russians.
I think the reason more advanced weapons are not being sent are not only because of capture/study reasons, but also training shortcomings, lack of NATO command and control structure, and just simple cost/quantity.
Many of the NATO countries are saying there is nothing left to give because they are giving reserve equipment (ie. Dutch APC's without guns, Germany giving DDR-era stocks). Any more giveaways, and you are basically stripping active units of in-service equipment. As mentioned by others, any NATO military that isn't the USA doesn't have huge standing armies. They are already stretched pretty thin as it is.
These companies are not set up for continuous war production, just batch to batch as they win contracts. With all the issues with logistics and staffing going on right now, it's not easy to "just make more" and of course the question of who's paying for it? (maybe the 40B aid bill)