1. Both Parent and Child Matrix's Display in Web Store must be checked.
2. Both Parent and Child Matrix record must have Online/Base Price set.
3. Transaction Item Option must be setup correctly. Check both Parent and Child record > under Custom tab > Item Options: make sure that correct Item option is selected on the field. To select multiple options, click on the Ctrl key + click on each of the options for the matrix record.
4. If Item Option is selected on the item record but matrix item is still not showing on the Web Store, go to Setup > Customization > Transaction Item Options > edit the item option > under Applies to tab > make sure "Web Store" is checked.
5. Under Sourcing & Filtering tab > make sure Source List & Source Form fields are properly selected.
If all of the above have been checked but item is still not showing, view related article "Things to check if Item is not showing on the Web Store"