From my experience, orchid seeds do not germinate like your typical seed, instead the start getting larger very slowly and turn green. Eventually when they are big enough to handle with forescepts or a very small spatula, you will see the beginnings of the first leaves. They will look like little points, and it will still be a long time until you see the first real roots developing. Personally I have lost some flasks from waiting too long to replate, so I have been replating when I start to see those first leaves start to grow.
Of coarse, I have not been growing many different species so your results may vary. You could always replate some and wait a while longer to replate the rest and see if it makes a difference.
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