In its (normal) figurative sense, means to be in a difficult situation where failure seems probable because of external circumstances. The phrase comes from cricket, of course, but changes in the game mean that this is now almost a dead metaphor; sticky wickets were a result of rain on uncovered pitches, particularly on badly drained grounds; batting after it had rained was extremely difficult because the ball bounced low and unpredictably on delivery - hard to score runs, easy to get out. In modern cricket the pitch is covered over as soon as it starts raining and when play is not taking place, in order to prevent this external distortion to the balance of a game.