You might want to check the configuration on both boxes. I don't remember all of the details (it's been about 2 years since I worked with a Foundry box), but there are different failover modes that Foundry supports on different boxes. It could be that you are set up with box A as Master and box B as failover standby. If you failover from A to B, it could be that once B sees that A is again available, it may decide to normalize things and switch back.
Allan