Dendrobium Nobile keikis not putting out roots?

[mexicowpants]

Hi, I have a pretty big dendrobium nobile which i bought with 3 canes -- one flowering, one a new growth and the third covered in keikis (around a dozen). I've had it around 6 weeks now, and the keikis are getting pretty big (the largest ones are around 20cm) -- but none of them have thrown out any roots. Is this normal? most of the pictures i've found of D. Nobile keikis have roots at a much smaller size, as did my other, smaller D. Nobile when it produced a keiki last year...

Any help much appreciated, cheers