If you kill one llama in a caravan, other llamas refuse to caravan with the first llama in the caravan, including ones in the caravan. Let me explain.
1. Create a caravan of ten llamas.
2. Kill the 3rd llama in the caravan.
3. The rest of the llamas, except for the one on the leash, and the second one, leave the caravan.
4. Attempt to recreate the caravan with new llamas.
5. The llamas don't join
7. Now it works!