File changes aren't ignored even when in the .gitignore file

If Git insists on checking in changes to a file even when the .gitignore file seems like it should be excluded, it is probably because the file is already in the repository.  Git will not ignore a file if it has already been committed.

To resolve this, open a Bash prompt in the root folder of the repository.   ( A shortcut for this in SourceTree is to click on the 'Actions/Open in Terminal' menu.   Run the following:

git rm --cached FolderName/MyFileName.ext

Then commit the change and push it up, and it should ignore the file from then on (assuming your .gitignore was correct to begin with).   Note that this will leave the file as is in your source folder.