Hello! I hope I'm posting this in the right place...

Okay, so, I have 3 batches of different papers that I've coated in silver gelatin and have been happily printing away on for the past week or so. The print developer I've been using was pinched from my University already mixed, so I'm not entirely sure what developer it is (but I assume standard ilford multigrade). I took a big container's worth and have been using half of it in a tray for the past week. It was getting a little exhausted, so I started using the rest of it this evening. Aside from being much faster than the first half, 2 of the 3 papers came out of this developer okay, but the other 1 has been consistently stained by the developer. This paper is a Somerset paper designed for inkjet printers and is considerably thinner than the other 2, which are bockingford watercolor papers. There really is no way this paper could be fogged, but that's the effect this new dev is having; the gelatin layer on the paper is turning grey as soon as it's placed into the developer.

Can anyone help explain this phenomenon? I thought maybe it could be a storage issue; the mixed dev has been stored in a plastic container for the past week. Or maybe it's just been hanging round too long? This is an odd effect for it to have though, isn't it? Only affecting one type of paper? Needless to say I'll probably be scrapping this dev and looking for some more, but I am still curious as to what's going on!