Firstly. Check out if you have any proofing issues holes in the walls, services entry points etc, and open up the manholes look for any earth being dug out , or visible smear marks, rat runs or burrows, check access to the house can’t be via bushes, trees and climbing plants, if your not detached check with your neighbour as well, if you have any of these try to sort out as best as possible
Trapping - the best bait I’ve used is a mix of used cooking oil, chocolate peanut and plain crisps put them all in a bowl and mix them up into a paste, if your losing bait and catching nought, it’s probably because the trap trigger set toooo hard, you can ease that by putting graphite ( pencil lead) on the bar. ( fens are notoriously a hard set trap unless you give em a overhaul to slick em up, fine emery on the and graphite on the spring runner bar helps)
The best rat trap I’ve found is the AF by Killgerm the trigger is very light and slick, ( even mice don’t get away on the rat trap) with these the bait port is a little pouch inside the plate the rat has to stand on it to smell the bait, the slick trigger does the rest
If you can attach some soft wire to the trap and fix it to something, you won’t lose any in the event of a mis catch
If you want half a dozen AF traps let me know by pm and I’ll pop some in the post FOC. I have a box full I got free with some monitors I ordered
Good luck
Phil