You don’t say where you got your seed from. Did you purchase it online or harvest from your own plants? If you are purchasing them online, check the reviews to see if people say that they had good germination.  If you took seeds from your own stock are you sure the seeds were mature?

The most common way to propagate hydrangeas is by cuttings. This way, you know that you will be growing the same hydrangea that the cutting came from. Many hydrangeas  are hybrids so will not grow true from seed and some of them may produce sterile seeds. 

Below is an article that may help you. 

https://www.torontomastergardeners.ca/askagardener/growing-hydrangeas-from-seed/