I have the same issue. It imports all bank transactions except for a span of 2 months and 5 days. It's incredibly annoying. I've clicked Update now multiple times but it doesn't do it. Did you find out what was the issue?
I appreciate you trying some steps to get this issue sorted out, starnroanise.
An interrupted connection between QuickBooks and your bank is usually causing this issue. Since you’re still unable to get these transactions, I’d recommend uploading them manually through CSV. This involves logging in to your bank, downloading the transactions from there and saving it in your desktop.