Is it planted in Full Sun? Sounds like either too much shade, pruning at the wrong time of year, or too much nitrogen. Forsythia is a plant that needs a lot of room (10' x 10')and should be left unpruned to let it have its natural vase shape.

With that said, the best time and way to prune a forsythia is in the very early spring during and directly after blooming. Prune it so you can bring in your trimmings and let the flowers open in the house for a nice touch of color.

Forsythia bloom on last years growth. If you prune now, you will be cutting off next years blooms, and I am truly doubtful there is time for them to grow back before dormancy sets in. This is probably the most misunderstood aspect of pruning. Shrubs which flower early in the year, on stems that they produced the previous year. Forsythia will begin next year's flower buds in June or July of this year.
When you do prune, cut back flowered growths at least 1/3rd of the way back into the shrub, to a new young shoot or node

Cut about 25% of the stems right back to within a few inches of the ground. This will produce nice lower growth, which in turn will be part of your framework for next year. If you don't do this, then the shrub will simply get taller each year, with the flowering growth right at the top.