Making yoghurt help

[SL]

Can anyone help me out with what I'm doing wrong? I have an Easiyo yoghurt maker (basically a big thermos flask) and I've been happily making yoghurt in it for quite a few months however the last few batches have failed miserably.

Normally, I use a couple of tablespoons of Easiyo mix plus UHT milk and leave to 'cook' overnight. The last couple of attempts, nothing's happened - it's just milk with bits of Easiyo in it. If I try to recook the same mix, I end up with massively split yoghurt - almost like a cottage cheese on top with whey (?) being the bulk of the mix.

I thought maybe the drop in temperature could have been a problem so I've been using it in the kitchen which is warmer than the utility room but this hasn't made a difference. My OH has admitted that he dropped the flask just before I started having problems but it doesn't look damaged and I can't think that simply dropping it would stop the yoghurt from cooking properly? Is the cottage cheese type stuff just because I'm overcooking the mix the second time round?

[Mobile Chicane]

My guess is antibiotic residues in a particular batch of milk.

Try organic milk and see if that makes any difference.