How odd. Has this particular female produced a litter and been successful with them before? Is this a particularily large litter numberwise? Sometimes you just get a litter that's not genetically as sound as other litters. That's why I'm so nuts about knowing which male and female pairing produces the best, hardiest feeders and sticking with pairings that work so well for me. If I get a litter that sort of not great I put that female to another male and if I get another substandard litter out of her, then she's out of the breeder group. Rats can be awfully inbred and sometimes you just can't avoid the occasional bad litter that can result from that. Your own careful management of the colony can avoid this as much as possible though over time.
What color/patterns are the male and female that produced this litter? How old is the litter? If still pinks can you see very visible milk bands? If older can you tell if they are defecating normally, any sign of belly distension or is it more just failure to thrive? Pics maybe might help illustrate.
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