Carp will pick up and spit out even definite foods, I have watched it happen with maggots, bloodworm, sweetcorn, boilies. Sometimes they come back and take them again, sometimes they leave them.
We choose foods by taste, carp don't have that recognition. What tastes good to us is not always what carp want, and what they like, I'm positive I don't think tastes nice; worms, maggots, slugs...
Add to that your hook, if it behaves differently from the free baits, carp (and other fish) will reject the bait. Carp and chub are specialists in the identification of hookbait. To some extent the more free bait, the less chance of them identifying the hookbait, or more correctly the pre-occupation they feed with less wariness.