help!! less slime coat at rear part of my sr body

Hi bros, i need help on why is my sr body rear part (near tail) has less slime coat, not shiny as the front part of the body. its been 1/2 years, try few stuff as adviced by lfs,

tried increase temp, put salt, put blackwater. all seem not help.

lastly someone told me its genetic. is this true?

all water param check and its ok. nothing wrong. ph is less than 7. no active carbon on filter.

any idea? how to get the slime coating back for the rear part of the body. as i looks wield. try to take the pictures, but its not clearly show. its visible on the eye. thanks in advance