New Herbalist Not Gaining Proficiency?

Hello! As the title says, I just changed my Herbalist today to a new wolf. I've sent him on a few forages (admittedly only 2) but I have seen no gain in his proficiency. With my old Herbalist, he always seemed to gain proficiency with foraging, so I can't recall if it took time for it to adjust or not. 

Herbalists don't always gain proficiency when foraging.

My herbalist's experience from random days this month:
20th: gained proficiency 4 out of 8 forages
15th: gained proficiency 7 out of 13 forages
10th: gained proficiency 4 out of 12 forages
5th: gained proficiency 3 out of 8 forages
4th: gained proficiency 5 out of 8 forages

So it looks like, at least in her case (I'm not sure what all would effect this), she gains proficiency from roughly half the forages she does.