The higher you have the water bottle the more pressure on the liquid going through the valve. This is amplified when using large diameter tubing and large nozzles on the valve inlet. I have found that this can help when you want drops to travel horizontally but (imho) it doesn’t really help if the drop is purely vertical (there is a lot of different opinion about this).
Too much pressure may give 2 drops.
What opening size does the valve have? The larger the opening the larger the drops will be and the slower it will operate, especially when using higher pressure.
Make sure you get air tight seals, otherwise you will have leaks and irregular drops.
Make sure the valve is working well. If the valve is similar to the ones I use you can take them apart and clean the inner parts. Make sure the spring is clean and working well.
If the valve “clicks” when switched, test it without connecting the water supply, just hold it and listen to the clicks. What is the fastest you can use it and it still clicks.