have you checked for bundles of feathers, pheasant carcuses ect. the shoot i used to attend had a problem like this and the keeper did abit of detective work and found out that birds of prey in the area were having good dinners and the stoats were also joining in aswell so next time you feed just have a little look round to see if you can find any evidence of stoats, birds of prey or even poachers that have been in the area. ATB with the rest of the season.