There are a couple of common reasons for forsythias not blooming. In very cold winters, forsythia flower buds can be killed, eliminating bloom the following spring. Sometimes all flower buds above the snow line are killed, whereas buds near ground level are unaffected because those stems were insulated by the snow. Two forsythias with greater flower-bud hardiness are 'Meadowlark' and 'Northern Sun'. These cultivars will bloom even if the temperature dips to -30-35 F. Improper pruning is another common cause of lack of bloom. Forsythia flowers form on the previous season's growth, so pruning should be done immediately after the flowers fade. Pruning later in summer or fall removes the next year's flower buds.