Ok guys I have a problem on tuning this car likes 20hz

[camshairtricks]

Ok this is my prob I’m having no matter were I tune my box this car seems to play 20hz pretty loud then Falls off again then at 28hz up to 30 it’s loud then has another flat spot at 31hz an up . I’m running 1 15 in 4 .200 cubes net after all displacements with 50sq inches of port tuned at 28hz was tuned at 26 but I switched it to 28 an that did nothing  but make the box bigger sounds the same was hoping it would get louder from 28 to 35hz but the thing still slams 20hz an didn’t get any louder? The car is a 2016 Kia Soul  box set up is sub up port back