When a check displays the terms, "payment in full," it is legally a "full satisfaction check" under the Uniform Commercial Code (UCC) Section 3-311. Unless you return the entire amount of the check to the insurance company within 90 days of the date that it was cashed, then you cannot recover any additional amount from the insurer.
I don't know in which U.S. Jurisdiction you're located, but every state except for Oregon has enacted the identical UCC section. So, more than likely, what I've just described applies to your circumstances.
In short, it's not too late, if you return the money you've received within 90 days of the date that you cashed the check. Otherwise, it's too late.
Assuming that you do return the money, then you can renegotiate your settlement based upon a new insurance adjustment. You may still have to fight with the insurer, but at least you have the ability to do so. Once 90 days passes, you are "toast!"
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