Child layers disappear when i link them to the parent layers.

Hi all, 

I am new wit animate and I looked all over the net i can't find the answer. 

I tried to connect my child layers to my parent layers, but for some reason some layers disappear when i link them. 

The timeline of the layers became grey. 

In the example below all the grey layers are hidden.

If somone as a solution please let me know. 

Thank you in advance