I just had the same problem. Are you getting a trickle of water continuously draining out of that bed? if so you are getting to much back pressure somewhere to start the full siphon and it is just acting like a continues ebb and flow. I discovered that when I raised my drain header a little on one end to make sure it was going downhill that I inadvertently raised the pipe under the grow bed so it was slightly up hill. Solution lowered the drain header a little so it would still be down hill but so the pipe coming to it was also a little down hill. I am using 3/4 stand pipe also with a 2 inch bell.