Think of IPS like it was a school subject. If you are learning maths slower than everyone else then when it is time to move on to multiplication you are still trying to get the hang of carrying the tens in addition. To catch up you have to go faster than everyone else, but that isn't possible. Now you have to leave addition and start doing multiplication, and you still haven't got the hang of addition, without which multiplication doesn't make sense. Pretty soon you are floundering in a mess of abstractions built on other abstractions that you don't understand, and it all goes downhill from there.
Of course there are remedial classes for maths, reading etc. They help a lot; sometimes some dedicated tuition can pin down the specific issue and deal with it. But there are no remedial classes for IPS; the slow learner is left to sink or swim on their own.
If you fail to get the hang of IPS when you start school the process is very similar to failing elementary arithmetic. You can't understand why everyone else is doing the things they are doing, or what they mean when they say things. So you get excluded by classmates during break, which means that you don't learn new IPS. And when you do catch on to something you do it wrong, or you don't realise that it's what everyone was doing last year, so now its considered childish. So you stay excluded, always wondering what the magic words are that will let you break into the social circle. Eventually you reach adulthood, and you still don't know this stuff. You are like someone who managed to get through school without learning to read (still happens, sadly), and now finds themselves being shouted at by the boss for not bothering to read the instructions.