Why didn't my hydrangea flower this year ?

the most likely cause is that you pruned them late winter early spring. If this is the case then you cut the flower buds out. both plants flower on last years wood.
don't prune them back this autumn or spring and then you will get them next year.

welcome to GoY

Just to clarify a bit, prune forsythia immediately after flowering and mophead hydrangea after the last expected frosts, just removing the dead flowerheads and a few inches of stem..