First of all, please try upgrading your Showkase site to the current version (v1.3.6).
This may or may not help with your specific problem but there have been several changes since v1.3.3 (please see the Version History for details) and once you have upgraded, we can be sure that we are not trying to fix a problem from an older version which might already have been addressed.
Instructions for upgrading can be found here.
If this does not help, then please post the URL to your Showkase page (or pages) which fail the Google Mobile-Friendly Test so that I can take a look and investigate further. It would certainly help to know what theme you are using, if you have made any modifications to the theme, what page types (e.g. 'About', 'Juicebox', etc.) fail the test and what custom content you have on the pages.
I have just checked the Sara Lee demo gallery (which uses the Kosel theme) and the ListViewer home page, the Gallery Index Page, the Juicebox pages and the About page all pass the test.
I have also checked the Paul Bica demo gallery (which uses the Boma theme) and all pages in this site pass the test, too.
Once you upgrade your Showkase site and post the URL, I should hopefully be able to help further.
Thank you.