How can i make you tip me?

i know a lot of sellers spend so much time on their gigs and they give a 50 dollar service for 5 dollars but don't get any tips.

i know this myself that on my drawings i spend from an hour and a half to three hours to make a drawing but i don't get tipped.

is there any way that I can make my customers tip me, a lot of my customers say they will tip me but never do :(