It works as it should. Only transforms respond to parenting. If you must animate opacity in unison, pre-compose the layers or use pickwhip expressions to link properties.
thank you for your aswer! But pre-composing does not help :-(
If pre-composing will not work for your design and you want the opacity of the parent null to control the opacity of the child you add an expression to the child layer's opacity property the is tied to the parent null's opacity. You can simply do this with the pickwhip by
- Select the parent and one of the child layers then pressing the t key to reveal the Transform>Opacity property of both layers
- Alt/Option clicking the opacity property of the child layer to add an expression
- Drag the expression pickwhip from the child layer to the parent layer
- Click anywhere in the Comp panel to enable the expression
- Select the opacity property of the child layer and choose Edit>Copy Expression only from the Edit menu
- Select all of the other child layers and paste (Ctrl/Cmnd + v
Your expression will look something like this:
That's all there is to it.
To learn more about parenting type parent in the search help field at the top right corner of AE and check out the help file. There are lots more things you can do with parenting than you think: Parent and child layers