That is a pretty wide open question as a lack of germination can be due to many things. Some plants don't set viable seed. Some seed require special conditions or treatments before they will germinate properly. Seed also needs to have time to mature properly before being harvested in order to be viable. Seed must be stored in cool, dry conditions to remain viable and hot, humid conditions cause them to decline rapidly.
If you can submit another question or several questions with each one mentioning a specific seed species I can do a better job at providing a specific answer.
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