New roots tip stop growing

[Pep]

I have a question concerning rew roots. It happens mainly with some of my cattleyas.
New roots start growing, white in the outside and green tips, and suddenly one of them stops growing and the tip change color to brown or black. The rest keep growing.
It can happen now or in summer, and temperatures in house never go lower than 18ºC.
I think that this happens even if i am not using fertilizer.
Am i doing something wrong?