Hello Sandra,
It is most likely a problem with the timing of prunings.  Forsythia needs to be pruned soon after blooming because flower buds for next year will be produced on the old wood shortly after blooming.  The site could also be too shady- forsythia will bloom in light shade, but really prefers full sun.  Another problem that can impede flowering is a very cold winter that can damage flower buds.  The following article is a good resource about forsythias: https://hort.extension.wisc.edu/articles/forsythia-forsythia-spp/
I hope this is helpful.  Thank you for contacting Ask Extension.
Ruth