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?

[estación seca]

That usually is caused by damage. It can be from an unstable plant moving in the pot, and the root is scraped by medium. It can be from low relative humidity. It can be from bush snails eating root tips. Fertilizer in the doses we use for orchids doesn't do this.

[Pep]

Thank you Estación seca, snails (i have found a few small ones in the sticks in the room at the garaje) eats the root and you can notice it. What you say about moving a newly potted plant is true, but it also happens with well stablished ones. It might be the same, i do move them when i take them to the sink evry now and then.
I will try to control in which plants id motr frecuent.