How to save Forsythia that do not seem diseased but dying?
If nothing has changed in that area (construction? irrigation?) then the likeliest explanations are weather and/or they simply are showing their age. Many of those stems are quite thick and old. My recommendation is to give these shrubs a major pruning to see if you can reinvigorate them. Approach it either of two ways:
1) (the less drastic approach) Prune back approximately 1/3 of the oldest, gnarliest stems all the way to the ground. The first few cuts will be difficult to get your pruning saw into the congested mass but as stems are removed, your task will get easier.
2) (the more drastic approach) Prune the entire shrub back to the ground and let it regrow from the root.
Read this article for detailed instructions: