Nature has it’s ways of keeping a seed from germinating at the wrong time, unfortunately, that may not be the same time as a gardeners timeline. Here are some reasons a seed may not germinate at the correct time, or at all.
Coating on the seed:
Seeds such as morning glories have a hard, waterproof outer coating. If you want to speed up the process of seed germinating you can file away a portion of the coating or nick the seed with a wire cutter. If you are afraid of damaging the seed, soak the seeds overnight to spring the seeds into an active germination mode.
Age of the seed:
Seed can be difficult. Immature seeds will not germinate and old seed may not either (although, I have had good luck). I find that whether you purchase seed or collect it yourself, it stores well if you place it in an air tight container in the refrigerator. Seed that I leave out seems to not germinate well, if at all.
Each type of seed has certain time requirements. Check an online guide or the back of the package. Some seed sprouts in a matter of days, other it may take years.
Have you ever noticed directly after tilling your garden that in a few days you have new crop of weeds? Some seeds need light to germinate. That is why you can not bury them too deeply in the soil. Other seeds do not like any light to germinate. Follow the planting directions exactly helps to ensure your seeds will germinate.
Seeds do need moisture but not too much for extended periods of time or it will cause the seed to rot.
Some seeds require a certain temperature to germinate. Some require a cool period and some like it rather warm.
Some seed need a cold period. The term for this is called stratification. Stratification is a cold period of 6 weeks at around 30 degrees. Some seed even need more than one period of stratification. That is a good thing to know if you are collection seeds to replant for next year.
If you are purchasing seed it should come with basic planting instructions. If you do not have instructions, there are plenty of online guides to look at to get more information on planting from seed. I suggest if you are unfamilar with the type of seed you will be planting, research it.