I've just been into GW Manchester today to buy some more paints (I had a good selection, but had a few important ones missing because they were left at the family home with TGSC) and mentioned to the staff about how the old Citadel paints (in the hexagonal pots with white/blue/black flip lids) are still really liquid, but the new ones are nearly dry after a year of being closed (the screw-top 'new' ones).
Apparently, before the screwtop lids, GW were asking the paint company for some improved yellows that would cover darker colorus better (if anyone has seen the old Bad Moon Yellow, they'll know what the yellows were like for not covering anything!) and the old suplier said they couldn't do it without making them toxic. GW then tried several other paints in the 'Eavy Metal studio, and settled on the newer ones which use water mixed with something else for the filler, instead of just water.
So, it's a new recipe with less water in the new paints, and so less water means they dry out quicker, and stupid clogged screw-tops make it easier for them to dry out, but the new Citadel paints should cover better - apparently.
So now you know