I have no idea because you didn't give any details or show us the properties of the layer. If you are talking about the purple line then maybe the layer is the wrong size, or there is a layer mask, but there's no way to tell without some details.
Select the layer that is giving you problems in the timeline and press the U key twice to reveal it's properties. If it is a solid then select the solid in the project panel so we can see the size. Tell us the size of your comp. With some details we can probably figure this out.