minishop2 - Products randomly not added to basket

Hi All

I have a seemingly random issue with a minishop2 site I've built:

Upon adding product to the basket/cart, I see the green ajax "added to basket" message, the cart quantity increases in [[!msMiniCart]] snippet, yet when I visit the page for the basket/cart - it's empty (yes, the snippet [[!msCart]] is there, uncached).

Has anyone else ran into this issue before?

Now the weird bit: It only happens around 40% of the time, the other 60% the products are successfully added, which makes this a total mystery to me! I've checked logs, but no issue in there.

TIA

Kyle