or didn’t want to try to understand something that was said

refusing to comprehend speech when that person is fluent in the language being spoken

You are either dealing with a troll(who would then use it to insult you)

Or

You are predicting or mindreading a person’s intention(which can bring your own frustration)

In each scenario you may have to make a judgment call on your own to spot the difference.

If you’ve tried several times to explain it; the person could be having an issue understanding it. Might it be a learning difficulty. They might be just as frustrated as you might become trying to explain it. They might even be visibility upset particularly if it’s a learning ability problem. At this point I’d say it’s ok to walk away or find something else to talk about and maybe revisit another time. Or don’t. Just stick to other topics. no harm in letting them alone about topics if they are too complicated and making them uncomfortable. Or find another way to even explain it that might even get through.

If however the person decides to insult you and misinterpret your words, find ways to harm you: you have a troll on your hands and it comes to them easily and they even smile while doing it(similar to siblings when they fight) but they take it too far. block or walk away, do not revisit. Avoid them at every chance they try to get under your skin. They abuse communication for fun and suck up your time.

If you’re around someone in a chronic case that gets worse as time goes on: They have a different disorder and not one as innocent as a learning disability. Cut off all contact. It’s an abusive relationship.

If it’s a sibling and it’s simply a jibe(doesnt get worse/short lived); feel free to develop some jibes yourself to get them back. Here’s one that has stood the test of time: “your face” It can be even healthy. Kind of like how pups or kittens play fight to try to train each other to deal with the real world. And sometimes it’s just good to laugh. Reminds that life doesnt have to be so serious and there is space to play in.

I think this is why language is tricky as there are many nuances to pick up on.

What’s the word for when you can’t think of a word? I used to know it…

I would still use illiterate in that case, and would argue that is the correct term at least according to Merriam Webster:

a : violating approved patterns of speaking or writing b : showing or marked by a lack of familiarity with language and literature

Or even just at its most basic:

having little or no education

Severe Auditory Processing Disorder?

“not able” and “refusing” are pretty different qualifiers

To me, “not able to understand” is a capacity gap where the listener doesn’t have the capacity or context to grasp the meaning as delivered

“Refusing” to me sounds like aggravated ignorance either by choice or by defense mechanism

  aphasia

  n 1: inability to use or understand language (spoken or written)
       because of a brain lesion

I don’t have an exact match to what your looking for, but a couple things come to mind.

Having “selective hearing” is joke condition: when someone fails to recognize that another (usually a partner or a parent) is speaking to them or fails to comprehend what was said, but is otherwise sufficiently capable of holding a conversation.

The implication is that they are able (or choose) to ignore questions and requests when they don’t wish to answer or undertake them.

“Willful ignorance” or “being willfully ignorant” is when someone refuses to engage in understanding something when given the opportunity or discounts arguments that challenge their presuppositions.

This isn’t exclusive to verbal communication but it does imply they are some sort of discourse that they are choosing to ignore.

Fluency in and lack of understanding of a language seem to be exclusive of each other.

Inarticulate?

If they can’t understand or express themselves in that language, they are by definition not fluent.

“Dumb”

If you’re referring to a literal inability to speak

It might not be used frequently, but perhaps “incomprehension”?

You mean the person can read and write, but is bad at voice communication? Maybe a hearing problem?

Not capable at all would be non-verbal. This includes vocal and non-vocal verbal communication such as PECs and sign. Non-vocal is not speaking out loud. Able to speak but not doing so may be called selectively verbal.

Not trying at all could have several outcomes and reasons behavioral and medical and would not be any of these. They may not learn to read so lack literacy or may not learn properly how to speak and lack fluency in their first language.

Edit: upon reading some of your other comments, you may be thinking of delays in receptive and expressive language. Which can have many causes.

It’s not really a thing is it?

I mean, language learners often have a disparity in their ability to read vs their ability to understand spoken language, but that’s mostly because of things like accents, speed of speech etc, all of which improve with practice/exposure…