The single biggest challenge to democracy is the voting systems we use. There are many different types, FPTP, STV, proportional, the list goes on, the one most of the world uses, and most simple way to run things, is FPTP. Whoever gets the most votes wins. Simple, easy, and it works. Correct? Not really. Here is an election in a fictional country called the Republic of Land. The ballot is simple, put a cross next to the name and party who you vote for, and whoever gets the most votes wins. The results look like this:
Candidate 1: 19%
Candidate 2: 19%
Canidate 3: 30%
Candidate 4: 7%
Candidate 5: 17%
Candidate 3 wins the election, the most people voted for him, out of the population of 1,000, 300 people said yes to him, but 700 people said no to him. This is a hurdle some countries have looked at, Australia has switched its voting system in some areas. but that's really the only notable example. The UK had a vote to implement STV, though the Conservatives and Labour ran a negative campaign, because they were winning because of the broken system. STV is mostly a ranking system, for a simple explanation there are many videos online explaining it.
This is the single biggest challenge for democracy; and we need to rectify it.