When a seed fails to sprout there is usually a simple and easily correctable reason. Here are the most common reasons seeds do not sprout along with troubleshooting solutions.
Soil is too dry. Small seeds sown very near the surface often fail from lack of moisture. They may have enough moisture to germinate, but then the soil becomes so dry that the sprout which is beginning to push out from the seed is dried up. Neglect that would do no harm to seedlings an inch or two high will prove fatal to seeds that are just germinating. Check soil moisture once or twice a day. Keep the soil evenly moist and not too wet. If you are germinating seed indoors keep containers away from excessive heat or drafts. Cover containers with plastic or glass until seeds sprout.
Covered too deeply. Seeds covered too deep may not have the strength to push through to the surface; they are weakened by the struggle. As a general rule sow seeds twice to three times their diameter. Check the proper seeds sowing depth on the seed packet. Tiny or very fine seed can be scattered on a potting or seed starting mix surface and lightly pressed into contact with the mix.
Wrong kind of soil. Ordinary garden soil, especially if it is at all heavy or clayey, is literally too heavy to cover the seeds with. Heavy soil has a tendency to crust when watered making it very difficult for sprouts to emerge. Seeds can not overcome the weight of the soil on top. A light, crumbly soil is best for starting small seeds. Light soil will not crust. Amend outdoor seed beds with humus or well-aged compost. Indoors use a soilless seed starting mix or make a mix using equal parts vermiculite, perlite, and sphagnum peat moss.
Soil temperature too high or low. The optimal soil temperature for seed germination is from 70 to 75°F. Check seed packets before sowing for the optimal germination temperature. Use an electric heating mat under seed containers or place the containers near a radiator, heat vent, or on top of the refrigerator if the soil temperature is too cool. Water with tepid water; cold water will lower the soil temperature.
Poor drainage. Use a light seed-starting mix and be sure germination containers drain freely. Avoid containers or flats with tight-fitting trays or bottoms which will cause the soil to stay wet and soggy after watering. Garden soil that is too wet or constantly damp will cause many seeds to rot. Be sure to amend seed beds with well-aged compost to make sure the beds are well drained. Water should soak down below the level of seeds not sit on the soil surface.
Careless watering. Water applied in careless spurts or streams can wash soil and seeds away or leave seeds uncovered and exposed or covered too deeply. As well, too much water may cause the soil to form a hard crust. For seed beds, use a watering can or hose attachment that delvers a fine mist of water. Keep the soil lightly and evenly moist during germination and after seedlings emerge. Check soil moisture once or twice a day and never let the soil dry out. Containers can be watered from below; set containers in water trays for 15 to 20 minutes and allow the water to wick up into the seed-starting mix, after be sure to allow the containers to freely drain.
Too old or improperly stored. Seed that is too old will not germinate. Check seed packets for seed vitality and expiration dates. (Seeds lose the strength to germinate over time.) Fresh seed not used this season can be stored for next season in a cool, dry place. Seed stored in a very warm or hot place will lose vitality and may fail to germinate the next season.
You can test the vitality of seed by sprinkling 10 or 20 seeds between two paper towels moistened with warm water. Fold or roll the paper towels and place one end in a glass containing an inch of water; cover the glass and towel with a clear plastic bag to keep the moisture in and place it in a warm spot. When the average germination time for that seed is up, unwrap the towel and count the number of seeds that have sprouted and divide by the total number of seeds placed in the towel. This will tell you the germination rate for this group of seeds.
Too much or too little light. Some seeds require light to germinate. Check the seed packet for directions. Seeds that require direct light can be sown on the surface of the potting mix and lightly pressed onto the surface. Some seeds require extra darkness to germinate; cover these seeds with ¼ inch of potting mix and then place black plastic or newspaper over the container until the seeds germinate.
Required pretreatment. Some seeds may require a specific pre-treatment: stored in the cold before sowing (cold treatment), soaked in warm or hot water (water treatment) before sowing, or a scratching or nicking of the seed coat (scarification) before sowing. Check the seed packet or a growing guide for pretreatment requirements.