imported mattes a few frames out of sync

Hi

I am working on a project where i have been supplied external mattes.
I import them into davinci then access them by right clicking the node of the clip i am working on
Most of the time they work fine, clicking into place, showing that the timecode matches

but on a few clips the mattes are clearly out of sync with the footage by a few frames.
I can see what needs doing... the matte neeeds to be slipped in time relative to its film.

But I cant find any way of doing this.
Does anyone know how to do it?

Thanks!!

Alan