Question
Hi.  We have a row of five forsythias bushes.  One bush is very
old, probably as old as the house, 50 years.  The other four I
planted myself about eight years ago.  One by one they have
stopped blooming.  It started with the old guy  a few years back.  
Then the one next to it and and then the next.  This year one is
in full bloom and the one next to it is looking pretty sparse.  The
other three are devoid of any flowers.  Each year they do cover
with leaves and grow like mad.  Just no flowers.  They are
located on the north corner of the house and get full morning
sun and partial sun in the afternoon. Aside from when I planted
them, I have not fertilized them as I assume that they can take
care of themselves.   I have checked out books but everyone
seems to think that forsythias have no problems.  Well, mine do.  
I hope you can help.  

Answer
Marsha,
The only question I would have to ask you is did you prune them back in the fall? If this is the case, then you pruned off the flower buds for this year. Forsythia should be pruned immediately after they are done flowering, as they set up their buds for next year on old growth, not new growth.
Other than that, the only other reason that I could find for the plants to do this is that the flower buds have been damaged by a late freeze.
There are certain pests that will attack forsythia such as root weevils and gall weevils, but these pests would not cause the plants not to flower.
I would suggest that you have the soil analyzed by your local cooperative extention for nutrient levels and PH. You wouldn't hurt them if you fertilized them this spring and again in the fall, but see what the soil tests say first.
I'm sorry I couldn't have had a definate answer for you.
Ed Gulliksen