The plug in things are pretty much a waste of electric
Carry on doin what your doing, trapping ,baiting ( what type of bait are you using) etc. Then look at proofing measures, they are getting in some where, and until this is sorted ur probably never going to cure the problem 100%.
look around the outside of the property, more often than not at ground level, it could be a crack in the pointing, old air brick etc that's allowing them access to the cavity and up to the loft. You'll need to be as thorough as possible with this in order to prevent access to them, if you can fit ur little