They don't bloom on new growth, they bloom on wood at least one year old. Because the buds overwinter if the winter is too cold for a particular kind in a particular area then no flowers the following spring.

Even if the rest of the plant came through.

Otherwise if they are declining for some reason there will be less bloom over time.

There are a few possibilities. Forsythia bloom less in shade, and if they are in a spot where they are getting an increasing amount of shade from other plants growing larger, that will influence bloom. If you have been pruning too late in the previous season, you may be pruning off the flowering buds (though I personally prefer the natural form of the shrub.) As Embothrium said, flower buds are winter killed at a warmer temperature than the leaf buds or stems, and in many parts of the country the last 2 winters (not this most recent one) have been unusually cold, so this may have killed the flower buds that in normal winters are fully hardy in your area.

You haven't said where you are, but it is useful to have that information as part of your name so when posting you don't have to remember to tell us. Here's how:

Go to Your Houzz in the upper right of every page, click Edit Profile, and on the left side click advanced settings. Well down the advanced settings page is a blank labeled Climate Zone for Garden Forums along with a link to find your zone.

If you add info on your state or the nearest large city you will get even better information since zone only relates to average coldest winter temperatures. You can do this by adding it to your Profile Information under First Name (Publicly Displayed) on the first page under Edit Profile.

Then return to the top of the page and click Done Editing.

In support of what Embo says above, it is fairly common following real cold winters here to see Forsythias blooming wherever snow was covering the branches, but if said snow did not cover entire plant, for there to be non-blooming areas. Completely normal.

in my MI .... F is subject to hard winter ... as buds were set the previous fall ... winter winds is my theory ...

out here in farm land ... its a 50-50 shots ... 7 miles in town [less wind] ... reliable ...

even across my 5 acres ... one blooms almost every year ... the other two dont ... which translate into micro climate issues ...

supported by the blooming below snow cover.. as the snow insulated the buds, from the winter winds ...

and.. another example of how i learned not to plant multiples of anything ... when one fails in some aspect.. they all can ...

if i were you ... i would remove one or two ... and try something else ... its time to change things up .... if you want to remove the irritation ... maybe quince ... spirea... etc ...

ken

thank you all. I am in 7b. This past winter was not as cold as previous years.