All I can really suggest is to experiment with different nozzles.
If the solenoids have been left for a while, make some test drops to get the liquid levels up before making the actual photograph drops.
I had a lot of issues with plastic nozzles, the brass ones gave better results but not perfect. I then started using small pieces of drinking straw and PVC pipe on the end of the nozzle which helped a lot.
Also check that you have an air tight seal between the nozzles and the valve (use plumbers tape). If air can get in the water will slowly drip out and cause missed drops etc.
- This reply was modified 3 years, 1 month ago by Martyn.