I would just hazard a guess, but possibly your PVC pipe attached to the bottom of the Bell siphon does not have a steep enough downward angle to cause sufficient vacuum to make it drain completely each time.

This could also be the reason it sometimes allows the grow bed to run over. The water going down the siphon pipe doesn't create sufficient suction to create the necessary vacuum to start it.

These are all contingent on you having the correct size pump to provide the right amount of water flow to the growbed to make the Bell siphon work properly, providing it is constructed correctly.

Not great answers, but as I said, I would just hazard a guess.

Need more info and photos.