Hey,
I created 3 credit notes for sales invoices. Two of them worked fine but the last one was created as normal:
But when I hit save, it has a zero value:
Please advise 
Can you show some more information such as the original invoice ? has it now got a zero balance?
First, @KIze, update your software. I can tell it is an old version because it is missing the dropdown selection box for selecting whether it is a custom credit note (the normal form) or related to an early payment discount:
This choice is explained in the Guide: Use credit notes for customer returns and refunds | Manager.
Also, please explain what your second screen shot is showing. Is that, in fact, an excerpt from the Credit Notes tab list?
Out of interest, I have a recent version 19.7.24 but do not have the Custom option described above
Have I missed a setting somewhere?
Hey, I am currently using the online version (paid subscription) - How do I update this?
The original invoice has 210.88 remaining. This is the same as the credit not balance but the credit note is showing zero.
The second screenshot if the list of all credit notes, the red box is showing the result of me processing the credit not. Thanks
It turns out this appears only when a sales invoice with a late payment fee has been created. I did not realize this until now.
You do not. You are always up to date.
So this looks as though somehow the entry did not complete. I suggest deleting it entirely and re-entering it. Let us know if that works. Based on what you have shown, it looks like you entered everything correctly. Of course, nobody else can verify that your customer and invoice number selection match those on the original sales invoice. Double-check those.