LINER STAINING

I HAVE AN ABOVE GROUND 15 x 30 AND DESPITE MY CHEMISTRY BEING IN BALANCE(PER A  CHEMICAL STRIP) I HAVE A YELLOW/“ORANGEISH” STAIN IN SEVERAL PLACES ON MY LINER.

I HAVE TRIED “METAL OUT” AND OTHER CHELATORS TO HOPEFULLY DISSOLVE IF A METALLIC PPT.

IF YOUR EXPERTS HAVE ANY OTHER SUGGESTIONS TO TRY I WOULD APPRECIATE  YOUR HELP.

THANK YOU.