Some candidates lose because they can't raise enough money, no matter how hard or skillfully they try. Others lose because their election districts are demographically wrong, because the trend is against their party or because their views are not close enough to those of the voters.
But many losing candidates could have won, if they had avoided making one or more of the following common mistakes:
Failure to develop in advance a comprehensive campaign plan, including a timetable and a realistic budget. In politics you can start late, but you can't start too early. Losing campaigns almost always misorder priorities, putting too much effort on things which can have little effect on the election outcome.
Managing their own campaigns.
Spending too much time at headquarters rather than going out personally to solicit votes or raise money.
Hiring consultants who personally absorb too much of their campaign budgets.
Spending too much of the campaign funds on paid media and polling and not enough on building an organization of large numbers of people in campaign activities.
Adopting (and sometimes changing) positions on issues because of pressure from major contributors or the results of public opinion polls. Polls can be useful to determine which of their personal positions on issues should be stressed in their campaigns.
Misreading public opinion polls, which usually measure preference but seldom measure intensity. Intensity, not preference, motivates people to act in politics.
Failure to stress properly the issues which motivate the core elements of their supporters.
Responding to every minor criticism rather than focussing on the carefully considered issue thrust of their own campaigns. Campaigns lose when too much on the defensive.
Failure to respond properly to continuing negative information, whether from an opponent, the news media or both. Ignoring a continuing negative issue won't make it go away.