I dropped Andy an email, and amidst his preparations for Salute, he found the time to answer quite quickly.
It's as Cait Sidhe mentioned; mould wear. As I understood it, they're in the same mould as the Hellhound heads, and those were 'mizing up' the mould.
He can still supply them, but has to cast them separately, as they're also out of stock. He was thinking of withdrawing them, as he mentioned that I was the only one who has ordered them!
Now, I know that mcfonz definitely had some, as my own alien horde project was heavily based on his, using the same heads, so that may not be entirely true. But obviously nobody has bought any since September 2017, when I ordered mine.
At any rate; I've asked him for about 40 extra heads, and he will cast them up. I told him that I was not in a hurry, so it might take a while, but if there are others interested in getting some, I'd suggest emailing Andy as well, so he can cast them up in one big batch...
Oh, and Onebigriver: if you're after Hormagaunt heads; I have plenty of spares...