Not sure how or what's going on, because I know I got the rub button active, but I noticed I'm unable to rub a tenants stomach at one of the larger sizes (I mean they are smothering the city right now...), but another tenant - who is also smothering the city, mountains too - is having no issues.
The only difference i can find is that both have not only different weight gain options, but also far different weight buffs on the respective area. (belly weight 314670 vs 4994 on the belly, with the latter being on max gain.) ((Putting the numbers down here makes this sound hilarious, with the smaller number being larger.))
The only solution I can come to is character A has such a heavy mass on their front, it's messing with the hit detection for rubs, which B lacks. I don't know how else to describe it, as this is the only difference I got.
edit: The only weird thing i noticed that I was unable to give rubs after giving the tenant the Happiness Booster badge, but putting that on a new tenant didn't impede it either.