Have a look at this thread I wrote a while ago.
http://www.findmeareptile.co.uk/snak...ding-tips.html
From what you have explained on your husbandry you don't seem to be doing any thing wrong.
But I'm a great believer that Snakes can sence atmospheric presures in the weather.
With the fact that we have had a Very cold, wet and long winter this year this would co-incide with the non feeding responce.
Have you changed your suplier of food??
Regards Deano...