Reduce your flow into the grow bed and see what happens.

Try a bigger air pipe to break the siphon or did you not use one at all?

Change the configuration of how the pipes that exit the siphon are. Remove everything so it just "drops" straight down. Do you have elbow fittings?

A picture would help us help you. There are so many ways to mess it up I can't just magically give you an answer based on that little information. Thanks!