Did it work prior to the update to Jigoshop 1.10.3? Try a few things to isolate the issue. See if it is interference with the theme by switching to Word Press Default like 2012. Then try. The next test would be to deactivate your plugins other than Jigoshop and Grouped Products Pro to test it that way.
This topic has been isolated as a PHP issue. Sites using Jigoshop now need a minimum of PHP 5.3 to run as new functions as of Jigoshop 1.9.6 require it. This has also been updated in the latest release of Jigoshop 1.10.3.