What "path" from where? You need to be more specific. Also you have not provided any proper system information, version info, comp settings etc..
Stroke is applied to a mask not a motion path. The motion path will be the size of the mask only when the layer is at 100%.
If you are talking about just applying the stroke effect to reveal a line drawn as a shape layer or a line drawn on another document then you can create the mask path on top of the line on the same layer and apply stroke. and set it to reveal original.
If you are doing something else show us a screenshot of your composition with all properties revealed for all layers. Just select all layers and press the u key twice.
Mylenium and Rick I appreciate very much your help.
My problem started with my Illustrator, for some reason I cannot open it because it crashes immediately on start up. I am trying to receive some help from adobe but so far I've been ignored - never mind this is another issue.
Because of that I cannot prepare my Illustrator file better to make my life easier in After Effects. However, I can use the Ai file I had already created. I am in a deadline and I have to animate this file, but my knowledge in Ae is still a limited.
All the lines I have to reveal over time are in the same Ai layer. Because of that I created a Shapes for Vector Layer, and pre-composed it so my file don't turn into a mess.
Screenshot 1 you see the main composition;
Screenshot 2 you see the pre-composition of the layer I want to work with;
Screenshot 3 you see that each path I want to animate is inside layers called "group", which are inside the "contents" of the layer "lines stock Outline."
Screenshot 4 you see the comp. settings. (I am working in Ae CC from a Windows 8)
I tried Rick's proposal for creating a mask path on top of the layer "lines stock Outline" and applying the stroke effect there, but I didn't manage to do it properly.
Is it the correct way to do it in this case?
Do you apply any transformations or other operations in the shape layers that may affect the appearance? Beyond that I'm still not exactly sure what you are having problems with. Seems all straightforward enough...
i am sure the only problem is my ignorance with this program. I thought it would be straightforward as well, but I don't know how to do it properly.
I don't have any other effect applied.
I am right-clicking on the layer and adding a New Mask, then if I put the stroke effect it affects the outline of the mask and not the lines I want to animate. I guess the question is how to apply the stroke effect on those paths that I showed before once they are all in the same layer.
Mylenium, I managed to do it! When I was following Rick's steps I didn't know I had to copy the path and past it onto the mask path. Thanks for the help guys.