Answer: Probably the most common reason for forsythia not blooming well is pruning at the wrong time and cutting off all the flower buds as a result. (The best time to prune is immediately after they bloom.)
Winter cold might be the cause. Often when you see forsythias with those golden ankles you will then remember a particularly cold winter blast when there was snow covering the lowest part of the plant -- which is also the part blooming now because the snow insulated and protected it from the cold -- the remaining buds having been winter-killed.
The other typical reason for forsythia not to bloom well is insufficient sunlight. These shrubs do best in full sun all day long. Soemtimes a location becomes shadier over time as neighboring trees grow bigger.
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