It sounds like it could be the same problem I ran into.
see this link: http://rayshobby.net/phpBB3/viewtopic.php?f=8&t=86&p=329&hilit=colliding#p329
Sequential only applies within one program, not between all your programs. You have to make sure there are no programs that have different valves open at the same time. There’s a way to check for conflicts suggested in the link. I believe having Sequental ON will prevent a conflicting valve from running. If Sequential is OFF, then multiple valves may run simultaneously (if I remember correctly). If you don’t want more than one valve on at a time, then you have to adjust the times. If you can have several valves on, then there’s not a problem if sequential is OFF.
Hope this helps.