Hi,
You're right that intermittent rules out the invoice ID collision: that one is deterministic, so every matching order would fail, not the occasional one. And since Worldpay does it too, and it has no PayPal invoice ID, we can set that track aside. What PayPal and Worldpay share is that both send the buyer off your site to the gateway after checkout, so the issue is most likely at that hand-off rather than PayPal itself.
On the confirmation page HikaShop auto-submits a form that forwards the buyer to the gateway. If that submit does not fire, the customer is left on your site with no way to pay, which matches what you describe. The most common device-related reason is the buyer being inside an app's built-in browser (Facebook, Instagram, Gmail...) instead of a normal browser: there the forward can open in a blocked popup and simply do nothing, which would be intermittent and look device-related since it depends on how each buyer reached your site.
To pin it down, next time it happens could you check:
- Does the customer stay on your site, or do they reach paypal.com and it is blank there?
- On the page they are stuck on, is there a "Pay now" button with a "click here if you are not redirected" message, or is it blank?
- Do the affected orders tend to come from an app's in-built browser rather than Chrome/Safari?
That will tell us whether it is the redirect being blocked, the page not loading fully, or something on the gateway side.