There could be many reasons why they are not taking to the rat pups. It may be scent related - try scenting your rat pups with mouse to see if that helps. It could be color related - if your snakes are used to seeing little white mice and your throwing in black rats they might be a bit confused. If they are used to mice darting around in front of them and the rats are just crawling across the tub then that could also be a factor. Although balls can live on an all mouse diet - it's a pain to have to feed six or seven mice to an adult snake than one rat - so try to get them converted before the habit really sticks. How are they eating otherwise to they eat their mice on a good schedule? Do they eat right away or is the mouse running around the cage for 20 minutes before the snake takes notice? Separating them could be beneficial also - but I won't harp on that unless you want me too. 
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Tosha 
"Nihil facimus sed id bene facimus"

11.42.0 Ball Python (Harry and Fluffy and gang)
1.0.0 Angolan Python (Anakin Skywalker)
0.0.1 Green Tree Python (Verdi - yeah I know but my kids love the book)
0.1.0 Bredls Python (Smurfette)
0.2.0 Feline (Pippen and Pandora)
0.0.1 Desert Tortoise (Pope John Paul aka JP )
2.2.1 Fish (1,2,3,4)
0.0.0 frogs rescued from pool skimmer