Polling companies
polls
It has become difficult to conduct polling with the advent of the cell phone, and the outrageous number of spam calls in recent years have made it even more difficult.
I don't think pollsters have been able to fix their problems from 2016. In 2018, there was one poll that had Andrew Gilliam
up by 7 points right before election day (FL governor), and DeSantis ended up winning. The other polls right before election day had DeSantis down by 4-5 points.
Part of the problem is the strength of the Democrat propaganda machine and its strategy of labeling Trump supporters and Republicans as racist. This makes Republicans more likely to hide their political views, especially from strangers (pollsters). I would trust voting data regarding party affiliation more than polls, but this is infallible, for example, approximately 30% of people at Trump rallies are Democrats, and may vote for Trump.
I know this is all stemming from Trumps 'fake polls' rhetoric, and I'm never going to convince you to not be a poll-denier, but...
The polls for the 2018 House, Senate and Gubernatorial races were over remarkably accurate. As in they got lucky. The average poll was within 2.8% with a slight bias for Democrats of 0.4%. And that included state and district polling which is far less accurate historically than the major polls you see in the days before a presidential election.
Looking at just the polls that were off the most and then using those handful of poll to calculate the reliability of the thousands of other polls doesn't really make sense. You have to look at all the results.