Problem getting Tag to Tag change to stick ie be saved to file

I have a situation where I wish to truncate the first 3 characters from the 'Title' field using Tag to Tag conversion. I have entered %TITLE% in the field box, and $mid(%title%,4,20) in the Format string box, and this has appeared to change '13 Unchained Melody' to 'Unchained Melody' but it does not save it to file. There is nothing I can do to make it 'stick' but I am sure I was using the same process recently for this application. I have also found that I can't use non-capitals in the field box. Can you help me resolve this?

It should only be TITLE without the %

In addition if you want to strip the leading number, consider using a regular expression that remove a number, irrespective of the actual length.

$regexp(%title%,^\d+ ,)

Brilliant - don't understand why I was using the % before and after the field title. It certainly works without them - but they have to be in the format field - right? Funny how the preview seemed to be working. Thanks a lot.

The %xxx% retrieves the contents of a field, the string.
The name without % refers to the name of a field.
So the answer to

is "yes" as you want to use the string that is found in that field for the treatment. You could also enter string constants and/or combinations of string constants and string variables.