I would recommend that you remove the discs and inspect them on the bench where you can get a proper look at the inside face.
For all you know the slots may just be filled with pad compound and you may find that they are not as badly worn as you think.
At the same time as CMS has said, check all the pistons in the calipers, however I do not think that this will be your issue.
Perhaps a case of "should have went to Spec Savers"
Alex.