Please note that QuickBooks Online also depends not only on the internet connection but also on the browser we're using. I have a few steps we can perform so we can resolve this.
The first is to reconcile your accounts through an incognito window. Another one is to clear the browser's cache. These are the junk files on the browser which can cause errors in QuickBooks Online. Lastly, make sure that the browser is updated. Old versions can also cause unusual behavior in QuickBooks.
I have here the articles you can check to learn more about reconciliations: