It's not uncommon to be left on your own and left to fight everything whilst two people in a hurry charge on ahead and don't bother to assist or revive. And yeah it's usually levels 31 and 32 that do it. Not all by any means as I have found some really helpful people who wait or come back to help and revive you. But it's very annoying and frustrating. Sometimes it's because they are real life friends or clan members who just look after their own. It being random I guess you just take a risk or get some mates to help out.