It is never a good idea to tee downstream from the walk in cooler. This set up is difficult to clean and may result in flow issues. However, this should not have anything to do with the FOBs malfunctioning.
Series should not be an issue either unless the are set up incorrectly. Big problem with series is if the are not emptied on a weekly basis. What will cause issues is if the bartenders are leaving the faucets open after the FOB has activated. Also, if 100% CO2 at the incorrect pressure is used, gas breakout can collect at the FOB allowing float to drop.
Scott Zuhse, Instructor
Micro Matic Dispense Institute