That's a stumper, especially knowing you haven't touched it in three years. Otherwise, I'd figure it was pruning related. The trick with forsythia, if you want to ensure you don't lose flowers, is to prune them right after they finish blooming in spring. You don't need a second shrub for cross-pollination, so that's not it. You might do a soil test to see if your soil is A) very acidic or very alkaline, thereby making some nutrients unavailable to your plants; and B) to see if the soil is deficient in any nutrients. In particularly, you'll want to make sure there is enough phosphorous available, because it is necessary for flowering. If it is lacking in the soil test, you can add it with bonemeal.
Luke Miller, editor
Garden Ideas & Outdoor Living magazine