If you've got one trapped, and it sniffs but doesn't eat, for whatever reason it doesn't like being trapped??? (Imagine that). You can try leaving the area, and letting it re-render but it may not work. My tribe and I ended up having to let it out of the trap, leaving to let everything re-render, and dragging corpses over to it (while it was running around free - so yes, mildly terrifying), then proceeding with the tame.

Pro-tip for feeding: one of the tribe guys is obsessed with breeding and mutations so we had some very OP rexes. We just cryoed abunch of babies, let it out (with cryosickness is easier), unclaimed and killed it, and it literally only took 1 baby rex to let us start riding. Then repeat to let the Carch eat the corpses while you ride, and viola. Your very own spikey giga.