If the overlap is not showing on preview, it won't show on print. First of all, you won't see the overlap on the first page. You will only see it on the 2nd page that has the same columns/rows. It may not be very visible on preview.
It is set on the Print/Print Preview dialog and is on the right column of the Grid Page tab. The default is 2 but it can be changed along with other options.
Remember that you make a default but any pattern you load will use whatever has already been set for that pattern, not your "default" if different.
The weird thing is, when I first created the patterns the overlap worked fine. But now none of the patterns, which all have a default setting of 2 for overlap, show any overlap. Do you think that the program might have acquired a corrupted file? If that is the case would it be possible to uninstall and reinstall the program without losing all the patterns.
An uninstall and re-install of the program should not lose any of your patterns. That is sometimes the way odd problems are restored.
If you are using personally create libraries, you may want to save them as "full" patterns to be extra safe but otherwise, there should not be a problem.