Wheelbuilding / Wheel Building / Wheel build help

My Open Pro rim 105 hub keeps unwinding (slowly) non drive side. d/s is 21 on the Parktool scale, 15 on non d/s spokes are dt swiss double butted 1.8.

Any ideas what could be cause, other than my shonky build skills? Front is true, done same miles since build on same bike.

tension sounds very low, I'd have it at at least 24 on the drive side. think my rear double fixed open pro wheel is 25 on both sides. thats with dt champ, I go pretty high because of my weight, the rim can take it, no signs of cracks at the eyelets. when its too high you'll know