Helen,
Keep in mind, you were having probbies from the start. Its' normal as the oncids age to drop of the older bulbs, its' just usually new ones take their place before that.
Perhaps it was just a weak grower?
Don't get discouraged, as a orchid grower this can happen from time to time. Sometimes we figure it out ... sometimes we don't.
I agree with you, I do think adult plants can be easier then the younger ones, their just not as forgiving sometimes.
Gin, Said it best .... Try again
Bag the last one and see what can be done for it.
Above all don't get discouraged, it will only hold you back from learning and enjoyment that will come
Rho, Don't you love when it comes together, you show off you!