So, I *think* the polling issue revolves around "shy voters".
To my knowledge, this hasn't been a probably historically, but in a nutshell there are some people who - when they receive calls / are interviewed by pollsters - don't want to admit that they are Trump supporters. These folks may not want to do so for a number of possible reasons:
(1) They are embarrassed about being Trump supporters, and don't want anyone to know;
(2) They are Trump supporters who are fearful of reciprocity in the form of threats, violence, etc. by non-Trump supporters, so don't want anyone to know;
(3) They are Trump supporters who knowingly and willfully want to mislead pollsters, thereby causing polls to underestimate their numbers.
Not sure how/if these issues could ever be addressed. I mean, this exact problem occurred in 2016, pollsters supposedly identified their issues, changed protocols, yet... here we are.
Polls still suck when it comes to measuring Trump's support. Doubt this will change, but hopefully, Donald's gone and never running for office again.