Can't copy a tracking mask onto another clip.

What all usual ways?

You cannot copy/paste a mask from say Opacity to another effect or attach it to the position of a still.

It all needs to be the same.

Not sure what kind of mask you were making, but let's say for example it's an opacity mask.  Create the mask, then highlight "Mask (1)" under Opacity in the Effect Controls panel.  Then in the timeline select the clip you want to copy the mask onto.  In the Effect Controls panel of that clip click on Opacity and then paste.