Are you fighting with other wizards, and are you watching which foes they are targeting? When you cast meteor you aim it at one specific foe, and it then hits all of them. But if the one you aimed at dies before your spell is cast (usually because one of your teammates kills it, but it can also be from fire elf or another damage-over-time spell), then your spell is not cast but your pips are forfeited. I try to aim multi-target attacks at the foe who is least likely to be dead by the time my turn to cast rolls around.
I agree, make sure you pick the one opponent as your target that does not die before your turn where the spell goes off. That is probably what is happening. I always try and target the ONE player with the most health and I also make sure I look at what the other team mates are doing so they don't kill my primary target and ruin my spell.
Unfortunately this is not a bug. It is how multi-strike attacks work. Susume (above) could not of explained it better. I too have had this problem multiple times due to other players not watching whom my Meteor Strike is assigned to. They don't realize (or know) that when you target the same one and you defeat them, it messes up the oncoming meteor strike. I tend to try my best to be the last person to place my attack just so I can try to make sure it goes through and my pips are not wasted.