The most common problem is unpleasant, strong odours which are caused by lack of oxygen in the compost due to overloading with food waste so that the food sits around too long, and the bin contents become too wet. The solution is to stop adding food waste until the worms and micro-organisms have broken down what food is in there, and to gently stir up the entire contents to allow more air in. Check the drainage holes to make sure they are not blocked. Drill more holes if necessary. Worms will drown if their surroundings become too wet.
Worms have been known to crawl out of the bedding and onto the sides and lid if conditions are wrong for them. If the moisture level seems alright, the bedding may be too acidic. This can happen if you add a lot of citrus peels and other acidic foods. Adjust by adding a little garden lime and cutting down on acidic wastes.
Fruit flies can be an occasional nuisance. Discourage them by always burying the food waste and not overloading. Keep a plastic sheet or piece of old carpet or sacking on the surface of the compost in the bin. If flies are still persistent, move the bin to a location where flies will not be bothersome. A few friendly spiders nearby will help control fly problems!
Source City Farmer