The polls will almost always be skewed in one way or another. Many news organizations that are clearly leaning to one end of the spectrum or the other will host a poll and make a news story about the results in their favor. Some will be skewed to favor Democrats, but other will be in favor of Republicans. The actual results may be a 50/50 split.
More democrats might answer the polls or not. It is hard to tell. Most polls are looking for a wide range of people and they will do what they can to atttact this range of people. It is also not clear whether Democrats and Republicans are split evenly or if there is more of one.
It is quite common for both parties to claim that they are adversely affected by skewed polls but it does not appear that that polling margins of error are significant enough to suggest deliberate bias. However, the sampling on some states often neglects minority voters which means that there is sometimes a larger margin for the Republicans than will be seen in the voting station.
Let's take a look at all of the online polls after the first three presidential debates. Hillary Clinton came prepared and Donald Trump stood on the stage slinging insults and failing to answer a single debate question directly. Most of the online polls after each debate showed Donald Trump as the winner, which was clearly not the case.