There is too little information to fully understand what is going on with your siphon.

First thought is that perhaps there is a little uphill run in your siphon drain pipe into wherever it drains back. You want it to always go down hill.

JCO's comment on the drain back height is spot on.

Here is what you can do to test your siphon timing:

Reduce you inflow to the point where your grow bed is full of water and flowing into the stand pipe but not siphoning. Then slowly increase the inflow (and I do mean slowly) to where the siphon starts to work. Measure the time it takes to break siphon. If it does not fully break (that all familiar sound) then you most likely have drain pipe problems.

Assuming it does break, then measure the time it takes to fill and start to siphon again. The total of the two times is the maximum cycle time your siphon and grow bed combination can be set at.

Now, slightly increase your inflow of water and measure the full cycle time. Keep increasing the inflow of water a little with each cycle until the siphon ceases to break and just keeps gurgling or starts to fill and then then starts to drain again before it completes filling. You have now reached and passed the minimum cycle time of your grow bed siphon combination.

Adjust your inflow rate to somewhere in between the maximum and minimum cycle times for best continuous operation.

Make sure you have a funnel on top of your stand pipe as it will increase the maximum cycle time giving you a higher minimum to maximum cycle time ratio and more range to set your timing.

Also, some folks set their siphon (bell) bottom cutouts very close to the bottom of the grow bed in an effort to drain almost all of the water from the grow bed. This is not necessary and can cause some restriction in the flow of water, especially when the water level gets low and there is less pressure from the water in the grow bed due to its low level. This is when the drain flow rate is at its least during the drain part of the cycle and, as JCO implies, the drain flow rate is set by the difference between the water height in the grow bed and the bottom of the drain pipe. That last bit of water is flowing the slowest and it might not be enough to maintain a siphoning action if the drain pipe bottom end is not low enough or is too large to prevent air from entering it and interfering with the last part of the siphoning action. Adding a 90 degree elbow to the end of the drain pipe will often help keep air out of it during the last part of the drain cycle.

I have found that cutting the drain holes (slots in my case) higher up (more holes) on the siphon covering (bell if it is a separate part from the shroud) helps in maintaining good flow throughout the drain process. It also allows for less restricted flow allowing for a quicker drain time. I only have a 4 inch shroud, which acts as a bell, with a removable rubber cap and a stand pipe with a funnel shaped reducer in the siphons I build, and they have always worked without problems. The shroud has horizontal slits cut from the point they push into the fitting (4 inch toilet flange that is held in place by the siphon bulkhead fitting) up to sometimes as high as three inches. The removable rubber cap allows for pulling out the stand pipe and reaching into the 4 inch shroud for easy cleaning.

I have spent countless hours on trying to improve on that design by adding tubes down the side of a bell (which worked for a while until it clogged) and in the end went back to the above design.

Oliver