Sidebar disappears

Hi,
We are having an issue where the right hand sidebar disappears on the shop category pages e.g. http://earthschooling.info/thebearthinstitute/?wpsc_product_category=earthschooling-curriculum

If we press the Flush Theme Cache button on the Settings > Store > Presentation page, the sidebars come back. A day or so later with no changes to the Store settings, the sidebar disappears again, showing a full width layout?

Can you please let me know if you have heard of this before and what may cause this issue?

Many thanks.

https://ww.wp.xz.cn/plugins/wp-e-commerce/