Just offer the rat of comfortable size or even a touch smaller and see what happens. If that doesn't work, try rlditmars recommendation. Then if still no action, you could try a live rat, and then switching from the live over to the F/T once the taste for rat is accepted/acquired. This does risk desire for only live, so question is what is more important to you, rat feeder even if live, or mouse feeder on f/t...
With any of these, patience is key, try the method a few times over a few feedings before moving on to the next option.
good luck.