580exii master 550ex slave

i recently recieved a canon 580exii flash and for the life of me i cant get the 550ex to slave off of it.. i set everything like i was suppose to.. funny thing if i make the 550ex the master and the 580exii the slave it works... i was wondering if im doing something wrong or is it not possible to slave the 550ex!?!?!?!?!? using it on a 40d please help!!!!!

Should work fine, but since I don't own a 550EX I really can't help troubleshoot.

I use a 580 1/2 master with a 550 slave frequently. Just make sure the little slider is set to slave and it should work perfectly. Put the 580 on master, 550 on slave. Make sure they're on the same channel (they're numbered) and set the master to A and the slave to B (fairly sure I have that the right way around). Point the 580 at the red panel on the 550, and hit the test button, they should fire about one second apart. If they don't either you've got a setting wrong or the 550 needs to be repaired.

You can't/don't need to set the master to group A. The master is always in group A.
But otherwise I agree. The only obstacle is if you set the slave to group C and the master to ratio A:B, then group C doesn't fire. You have to set the master to no ratio (A+B+C) or ratio and group C (A:B C) to fire a slave in group C.