Track Motion "Apply" button is greyed out

I've been using the track motion feature for a few weeks successfully. Now every time I try it will track a point through the entire clip but when it reaches the end the "Apply" button remains greyed out. Its possible my brain cells are playing up....

Workflow includes:

import mp4 video

create proxy to jpeg sequence (video)

run the track motion with 1 tracking point

tracking runs through all of the frames

then nothing

screen shot attached - any suggestions appreaciated!