Are you sticking witht he same prey type (mice) or looking to move from mice to rats as well? Your best chances of success will involve only making one of those transitions at a time.

When we were bent on getting all the critters on f/t we did not have too much of a problem though some required we give them the f/t mouse-jig puppet show.

THe thawing process can matter as well - if the f/t prey gets too wet (condensation, leaking bag) it seemed to dilute the scent somehow.

Also making sure they are warmed up enough they provide a good thermal image is key.