If your reaction did not work, ask yourself these questions:
During Setup:
Are you sure the reaction was run in clean glassware, with pure solvents, under an inert atmosphere?
Were your calculations correct?
Was your starting material pure?
Are you sure your reagents were of good quality? (some reagents should be distilled immediately prior to use in a reaction: see <should it be purified?>)
Did you run the reaction at the correct concentration?
Running the Reaction:
Were you able to monitor the reaction accurately, or was TLC unreliable? (see troubleshooting TLC)
Did you monitor the reaction from start to finish, to see when the problem occurred? (e.g. did you run the reaction for too long, so that decomposition occurred?)
Were you aware of the temperature of the reaction mixture throughout the reaction course? See notes on temperature.
Did you add the components of the reaction in the correct order, at the correct rate? See notes on adding reagents.
Was stirring efficient? If you noticed that stirring was problematic, see notes on stirring for some tips.
If no reaction occurred, could you raise the temperature, or add more reagent, to encourage reaction?