Like you say, if it's new to them they don't touch it. This behaviour is called neophobia. I have put peanut butter mixed with olive oil out on the bird table and the rats will go straight to it. The Mrs puts such a varied amount of food scrapes on there that they don't take any notice of what it is. I did a rat job in the attic of a foody pub where I screwed down 6 Fenn traps with peanut butter on and they wouldn't touch it. Checked every day and nothing. Put a trail cam up there and the rats were coming up to the traps and even putting their noses a few millimetres away from the bait, give it a good sniff, but wouldn't take it. In the end I used 2 trail cams to see where they were going in the attic. I could see where they were walking across the rockwool insulation. Put a Fenn on top the insulation then cut around it so the trap, with no bait at all on it, fitted nicely in the hole. Then covered the trap with a thin layer of rockwool. Turned out there were only 3 rats in total but I caught them all this way....booby trap!.
I maybe wrong , but i though even in a loft a fenn needs to be set inside a tunnel , artificial or otherwise . Just what i've always practiced would be nice to know for sure.