It is so difficult to diagnose problems at long range, and there are so many factors necessary for good growth especially of seedlings. There is a very good book just on that subject - probably out of print, but maybe a good Orchid Society library will have it - "Orchid Seedling Care" by Bob Gordon. I can't recommend it enough. I bought it, newly published when I was struggling to build a paph collection , and importing half a dozen flasks at a time from a New York state specialist, whose name I now forget anyway ( I stopped buying from him, when he changed his agar - or the lab he was using changed - to a slushy one which did not hold the seedlings in place firmly, so that after an air-mail journey across the atlantic, I opened the flasks to find a sort of gooseberry jam of seedlings and mush...) but before that, using Bob G's ideas, in the best cases, I got up to flowering every one in the flask, within 2 years from deflasking.