re: "I’m still being charged 20% on total earnings between $500-$10000. Can someone kindly explain this."
If you have earned more than $500 while working for the same client, then the Upwork fee is 10%.
After earning $10,000 with the same client, the Upwork fee drops to 5%.
This happens automatically for all freelancers. You don't need to do anything special for this to happen.
re: "I’ve been with this client for over a year."
The amount of time you have been working for a client is not a factor in when the Upwork fee changes based on earnings with a client.
I checked and can confirm that the service fee was applied correctly for the payments you are referring to. You can check the last few invoices (Service Fee Invoices under your Transaction History) you have with this client to find that you were charged a 5% sliding fee for earnings above $10,000. Please let us know if you have any further questions or you need our assistance.