Why do the buds on my Moth Orchids grow to about 1 to 2 cms dia only to drop off before opening

This is usually a symptom of either drought or poor drainage--both reduce the amount of water getting to the top of the plant. I have also seen this with extreme lack of light, but ordinary lack of light usually reduces or stops the formation of future flowering spikes.

I'm not particularly expert on orchids but at the moment I'm having great results with mine after years of effort. They can get stressed, get too wet, too dry, too hot (don't stand them in too sunny a position) and are a pain, but well worth the wait for flowers. I've been dunking mine in water for about an hour every week with the addition of orchid in bloom fertiliser and all three are in bloom. This is my way of doing it and I know there are a lot of orchid growers who do it differently.