Correct results only displaying for first search

When the page first loads, you click on a category in the sidebar, the correct results display. If you try to click a different category in the sidebar after this, it reloads, but it is the same results as the first search.

Sometimes you can click a few times with the correct results, but inevitably, it ALWAYS stops working and just reloads results.

Has anyone here encountered this before? Can anyone help me troubleshoot?

Thanks in advance!

The page I need help with: [log in to see the link]