OS X :: Unable To Retrieve Lost File / Backup Created Using Time Machine

I have a file I alter once about a week, each time I alter it I save it as a new one, with a date included in the name.So I have a series of files something like

xxxxx march 13.numbers
xxxxx march 07.numbers
xxxxx march 01.numbers
etc.

This morning when I updated and saved the march13 file I accidentally saved it as xxxxxx march 13, not xxxxxx march 20 which it should have been. No problem, go into time machine and retrieve the march 13 file I thought.Its not there. Its not there from just before the save to any time it should be there !

Luckily I was able to retrieve it as I also have a once a week online backup so I went into the last one of those and retrieved "xxxxxx march13" which proves it did exist, it has a creation date of march 17 but if I look at TM for march 18 its not there - why isn't it in my time machine backup?

All the other files in that folder are as expected so the only thing that makes any sense to me is that TM hasn't been backing up over the past week or so but TM error logger shows no errors.