So, from what I've understood, pets are supposed to gain skillpoints when leveling up depending on diet, right?

I'm not sure if I have a glitch or if I've completely misunderstood, but...

I was trying to level up my pet, 8lue8lood, to get its Constitution over 100. It was 99 skill points when on level 31, and I made sure its diet composition had full five "stars" on Plant Fats [since that's what seems to affect Constitution - I've had it like this for several levels]. So, when finally reacing level 32 after several days' experience gaining... I discovered it still has 99 Constitution. Am I doing something wrong, am I failing to understand basic game concepts or...?

I believe there is an element of randomness involved even after you adjust the diet and you just got unlucky this time.

As Lytala said, there is still randomness in there, even with 5 stars. The 5 stars make it much more likely that you get a point in that stat, but it's still not 100% guaranteed. Sorry it didn't work out, guess all you can do is wait for yet the next level - or feed your pet a Cactus Melon. This raises Constitution by 1 to 3 points, so would definitely get you to over 100. 

Alright, thank you two... I guess next time I'll arm myself with four-leaf clovers to get that stat up. ::::')