I believe that hydrophobicity, if that's a word, is actually measured by the contact angle of the beading...the higher the contact angle the rounder the bead. Most of us that have waxed a lot of cars have had the same experience as the OP, where the beading changes over time, and take that as an indication that the protection is less than it was originally.
As we have learned over the years, in a lot of instances the protection is still there, however it has become "clogged" by contaminants. In this case, it could be contaminants washed down with the rain, such as from overhanging trees, etc. Of course, as someone on another forum many years ago used to say, the best way to get that "just waxed" look...is to have just waxed the car.
Heijneker, you might want to think about Gloss-Coat or another coating, or using ONRWW to get a little fresh wax with your wash.