The browser's Developer Tools>Console area might shed some light on what is failing to connect. You'll want to open that then refresh the page and let the error present itself again.

Thanks Kyle, i'll get them to check tomorrow.

They also have a lot of IP connections to their accounts compared to any other clients.

One of them is showing 82!

82,63,45,44,18,15

Compared to the next closest client which is 8

Is that normal or could that be part of the problem?

That could indicate they have cloud storage connectivity or some other integration to the account present that is failing. Normally when I see a large list of connected IPs I suspect attached email clients and third-party services most likely.