I am trying to apply a customer credit to an open invoice, when I have applied it, it has reduced the amount of the invoice still owing down but the credit is still open.

Hello, JHL Brick Cartage.

I'd like to help you get this sorted out. 

To clarify, do you see an unapplied status for your adjustment note? If so, this will remain open if there's still a balance left to be applied. Please make sure to check the open balance under New Transaction in the Customer Information page. Here's a sample screenshot for a visual reference:

If the credit is successfully applied to the invoice payment and there's no open balance, try to delete the Receive payment. Then, re-create the transaction and apply the credit memo.

I've got this article for the instructions: Create and apply adjustment notes or delayed credits in QuickBooks Online.

Otherwise, let's perform some troubleshooting steps. This will help us isolate any issues related to your browser's cache.

To begin, open your QuickBooks Online account in an incognito or private browser. This mode will not save your browsing history and will also help isolate any web issues. 

Here are the keyboard shortcuts:

• Google Chrome: Ctrl + Shift + N
• Mozilla Firefox: Ctrl + Shift + P
• Safari: Command + Option + P

Next, go to Sales > Customers and select the customer's name. If the transactions are Closed in status, go back to your regular browser and clear its cache. Clearing cache fixes certain problems, like loading or formatting issues on sites. It also helps pages to load faster.

Finally, restart your browser to update the settings. You can also use a different supported browser. 

Please let me know how things go after trying the steps above. I want to make sure this is resolved for you. Stay safe!