BTA's can only handle food soo big, when it's too big they very often either take and hold it like you described, or eat it but then spit back out before fully digesting it.
I used to feed medium shrimp from the grocery store cut into chunks. A lot would be spat out or dropped.
A couple of yrs ago Anthony Calfo spoke at a meeting and advocated feeding much smaller stuff such as mysis shrimp because of the above mentioned spiting out/dropping issues. I gave it a try and all I can say is that my RBTA's have done at least as well as before just being squirted with mysis as I feed my fish. It's a lot less messy and IMO works better. It also is one less thing to deal with.