No parenting skill for taking care of infants?

I did just notice Parenting skill went up when I washed my infant in the sink, but that's been the only interaction I've seen so far that affects the skill.

I noticed the same thing - I haven’t tried washing them in the sink but certainly skilling or feeding them doesn’t give any parenting skill. 

I just expirienced the same, but can't confirm it from an old savegame I played before. I'll try to test some more tomorrow, but as it's been different to my last household, there are two options where it might come from:

1. My actual household is the first one I'm using mods - I'm using quite a lot of CC furniture and also some script mods (MCCC & Woohoo, Slice of Life, Meaningful Stories), so it might also be caused by one of the mods or a setting in MCCC

2. There are further sims in my household, I'm not sure if it's only not working for other members of the household  who take care of the infant but also for the parents

As I don't own the growing together extension it must be related to the parenthood DLC.

I've checked with a full household where all five YAs and As (it's an Apocalypse Challenge and I haven't lifted the restriction on moving them out yet!) are either the parents or the infants have been made care dependents. Parenting skill for all of them rises only when the infant is being bathed or washed. Nothing else brings up the skill bar or changes the skill percentage. If it's happening for one task it looks as if it's an oversight that it doesn't rise for anything else – I wouldn't have thought bathing is more important than the other infant skills.

I've also had an eye on it today again:

Feeding with bottle brought a little progress for the mother's parenting skill

All other interactions like talking, caressing, change diapers, bring her to bed and so on had no impact on the skill.

I'm still not sure if it might be related to one of the mods I use as in my last vanilla game that I started about a month ago there was, as far as I remember, a constant progress for any interaction from newborn to kid and because I set the lifecycle to long it built up annoyingly fast.

I am having this issue, but only with one of the parents. The mother is increasing it by doing anything with the infant, but the father isn't increasing at all. She's at level 6 in parenting and he is at stuck at level 4. At first it was because she was doing most of it with breastfeeding, but then I noticed the discrepancy and started having him do everything else, and it stopped increasing. Anyone have a fix for this? 

Update - 2 days later - I noticed that actually its similar to what some people have mentioned above - its only when doing 1 or 2 interactions - the mother is only getting parenting skill when feeding the infant (but I could have sworn it was more interactions than that just hours before posting this, and I have no mods/CC so the only thing I did was add a second floor to the house), and the father only gets the increase with 1 interaction but I can't remember which one... Oddly it wasn't bottle feeding or experimenting with foods in the high chair. 

Is it possible that there is a cap on the amount of XP they can get doing certain interactions, so you don't spam an easy one to level up fast? And is it possible I met the cap for the other interactions? Or that you can't learn all you can about parenting from just the infant stage? Because I also so somewhere that you can't use books alone to fill up on parenting ("because you can't learn all you need about parenting from books" - which I honestly love) so maybe they set it up that way with the life stages too? Maybe we need to interact with the children to fully get more skill increase? 

I will say, I felt it was insane that they leveled up to 3 or 4 just caring for an infant, and they were unlocking all these fun things that you can do (negotiate with your kid to clean, discipline them, give time outs...) and you can't do any of that with an infant, so it felt a bit OP that we were getting an advantage going into the other life stages....  

I was extremely happy to be able to make my sims have a kid but then when my parenting skill didnt go up, nothing made it go up! Cheats, changing, feeding, washing ect! 

Yeah, from what I've observed, SOME infant interactions seem to increase the parenting skill , like giving the infant a bath or washing in the sink, and feeding with a bottle or breastfeeding, but basically nothing else. Feeding at the high chair doesn't do it, which is a real bummer, but biggest disappointment is that none of the "milestone training" interactions like tummy time or practice crawling do it. Which is weird, since I'm pretty sure toddler interactions like "teach to talk" makes it go up, that those are kind of the infant equivalent to that so...yeh, idk.