Interesting. It is likely that the Bob Betts seedlings are tetraploid like the parents must be. Tetraploids can be slower growing, but I suspect that the other parents in your crosses are probably also tetraploid to be as good as they are. The only other thing I can think of is that Bob Betts has a lot more mossiae in its makeup (about 2/3) whereas the other crosses are all under 50% mossiae. Mossiae has very minimal rain in the natural habitat Jan/Feb/Mar getting only some dew and mist. That might make some Bob Betts are little more sensitive to constant high moisture and nutrition. Realize that I have never grown a flask out, so what do I know!