[Tania]

all my other melons are up, but this one is very stubborn and doesn't want to show up; it is Day 20...

Did anyone noticed if it takes a long time for this variety to germinate?

The seeds are fresh from Baker Creek 2007, I started 6 seeds, and I have to confess it is a bit cool in the laundry room where I start seeds, temps are around 62F at night and 70-76F during the day, and no heating mat.
However, other (older) watermelon seeds already sprouted a few days ago (worst case was 15 days).