I have tried PETG twice (different spools) with the same odd results - the first couple of prints come out perfectly. Then all other prints refuse to stick to the bed. I have been really confused by this as it is happening with the exact same gcode as the good prints.

I first tried with blue PETG, then white PETG. When I switch back to PLA Pro everything works fine.

This is what I was printing http://www.thingiverse.com/thing:795954 I printed one set just fine in PETG, and then tried to print a backup and never could get it to stick.