After many months of working normally, my 550 EX won't go into E-TTL mode on my 20D--not matter what I do. I put the flash on my Elan 7 and it goes back to E-TTL. Take it off, put it on the 20D, and it goes back to TTL, and switching modes on the flash unit does not bring up E-TTL. All I get now are blasted frames. I've cleaned the contacts, fiddled with switches, etc. No E-TTL. Thoughts?