Can't delete file in Time Machine backup

When I try to delete a file from my Time Machine backup (by right-clicking on the file within Time Machine and selecting the "Delete All Backups of filename" the pop-up box that usually shows next to confirm, is hidden behind the file listing on my screen (I can just see the very top of the box but with no text). So I can't go any further to actually delete the file. This is a recent phenomenon and I have no idea why. Any ideas? Thanks