There are a number of factors that can result in poor seed germination. A suitable medium should be used for sowing in trays and pots and there are many very good peat, coir and soil based composts available.
For outdoor sowing the soil should be warm, well drained and suitably cultivated. Moisture and air content is very important and seed failure can be caused by either the soil or the compost being too dry or too wet. A very wet growing medium contains very little air and under these conditions the seed may rot. If too dry, the outer seed coat does not break down which also results in failure.
Another of the more common reasons for germination failure is sowing seeds too deeply. When you do this the seed runs out of energy before the shoot reaches the surface so make sure you sow at the depth stated on the seed packet. It is important to note that some seeds need light to germinate and these should be pressed into the surface of the compost. Also bear in mind that some types of seed germinate in a few days whilst others can take several months.
Each seed variety has an optimum temperature at which good development will take place. Temperatures that are too high are just as likely to have a detrimental effect on germination as temperatures that are too low. It is important to follow the guidelines on the seed packet and don't be tempted to sow too early when temperatures may be too low or difficult to maintain as the soil may still be cold and wet.