Is there a word/phrase for "deliberately failing to understand a concept being explained"?

I would use ‘abstruse’ which means ‘deliberately not understanding’.

Abstruse is perhaps best understood from its etymology - it means ‘to push away, to hide’ in Latin, and is the word I have most heard in spoken English conversations (I am English) to mean ‘deliberately not understanding’, or ‘making a thing of not understanding’. Online definitions of the word don’t capture its meaning very well - they say it means ‘esoteric or unclear’ but it means more ‘pushing the meaning away’.

Here’s an example of it in use:

‘Sir! You told me the washing machine would arrive on Thursday and be fully installed!’

‘Madam! You are being abstruse - we clearly told you it would come with a box of bits you have to install yourself!’

Note: when you accuse someone of ‘being abstruse’, it carries the meaning of ‘deliberately not understanding’ - often for the person’s own gain (in this case, she doesn’t want to set up the washing machine herself).

Whereas ‘an abstruse mathematical formula’ would tend to mean ‘difficult to comprehend’.

Definition Abstruse: ‘Difficult to comprehend’ https://www.merriam-webster.com/dictionary/abstruse

Etymology Abstruse: https://www.google.co.id/search?q=etymology+abstruse&ie=UTF-8&oe=UTF-8&hl=en-id&client=safari

Another option is ‘obtuse’. This literally mean ‘off at an angle’ at an oblique angle - to the thing trying to be understood. It carries more of a sense of ‘being bone headed’ or just ‘not getting it’ than the deliberately obfuscating nature of ‘abstruse’.

Example: ‘When you say that you can’t simply put the key in the door, and open it, now, because someone else should be there to open it, you are being obtuse’.

Oh and then - obfuscating could also be good - it’s a verb that means ‘deliberately trying to hide the truth’. Putting obstacles in the way of the truth.

But abstruse is the most accurate for what you are looking for I feel.

Obtuse

ob·tuse /əbˈt(y)o͞os,äbˈt(y)o͞os/

annoyingly insensitive or slow to understand. "he wondered if the doctor was being deliberately obtuse" (Google)

or

not quick or alert in perception, feeling, or intellect; not sensitive or observant; dull. (dictionary.com)

Obtuse is the way some is acting. They may not understand; however, when all evidence is to the contrary, they my be "deliberately obtuse". The difference is why it is happening.

So you're just being deliberately daft, got it.