I'd definitely recommend getting a credit report (not from the websites that advertise with an insane jingle, but from the actual credit bureaus

you're entitled to a free report). Mine had debt from a relative with a similar name; I was able to get that removed. They will also tell you in more detail what goes in to calculating it.

I agree that it's not perfect, and often very opaque, but you should be able to get some understanding of why she doesn't have good credit.

Does your mom have debt that she pays on time? Is her “doing everything right” visible to credit scoring agencies and aligned what statistic says about good borrowing customers?

Credit score doesn’t mean “runs a good personal economy” it means “likely to pay their loans on time, consistently, based on statistics that are observable”.

and have no idea why

Just because you refuse to learn doesn’t mean it’s magic. It is very simple to understand why exactly you have the credit score you do. Maybe mommy isn’t being entirely truthful with you.

Most people who think they understand how credit scores work...don't understand how credit scores work.

The biggest things are loan-to-limit, payment history, and average age of accounts.

Loan-to-limit is easily achieved by keeping balances below 50%, and ideally below 30%. It's also helped tremendously by not carrying a revolving balance (paying the statement balance in full each month) and not closing idle cards.

Payment history is of course helped by making payments on time.

And AAoA is probably the easiest. Just don't close cards. Call and "downgrade" a card if it isn't worth the annual fee. If there's no annual fee, there's no reason to close a card.

Just make sure you use it every now and then and pay it off. I sock-drawered one of my oldest cards a long time ago and it just closed last month from being idle, and that took a hit to my score (high limit gone and it's no longer incrementing time in my AAoA).

It's also worth mentioning that credit scores don't matter until you are looking for credit. Credit cards are probably the easiest way to build credit, as long as they are used properly. But they'll give a basic card to any schmuck. Where it really matters is getting mortgages and larger loans like cars. That's where having a good score matters. And also better cards that earn more points/miles/cashback and have other fringe benefits.