Mobs despawning nearby

I've been trying to get the sniper duel achievement but before I get far enough away the Skelton instantly despawns, I know it's not it just not being rendered because when I go back it's still not there, for more detail, the Skeltons are put in a boat (this was how I safely removed it from the mob farm) an kept in it also my device isn't the best(with the recommended render distance being 6 chunks)