I want to grow lettuce. I used fresh Grand Rapids variety seeds but the problem is that they would not germinate. Day temperature there is between 26° to 29° C (78° to 84° F) and night temperature is between 19° to 23° C (66° to 73° F). Now, what is the reason that they would not germinate?.
Lettuce, while now bred into dozens on varieties, originally comes from the Mediterranean, an area with hot and dry summers. A mechanism called thermo-inhibition prevents the seeds to sprout when the soil is warmer than ca. 27C / 80F. In the wild, this protects the seeds to grow in conditions when they have virtually no chance of survival; for a gardener, this means germinating lettuce in summer often can be difficult.
Air temperature is less critical, btw., but remember that soil in direct sunlight can get really hot. Plus this can mean the top layer of soil (and the seeds) is prone to drying out.
Some lettuce seeds need light to germinate, so sowing too deep can also be a problem.
For temperatures that are just a bit too warm, soaking the seeds in cool water for 16-24 hours can help overcome the temperature inhibition.
For more reliable and temperature-independent results, consider germinating your seeds in wet paper towels and then planting the tiny seedlings. Of course that means you must protect them from the summer sun and keep them constantly moist during the initial growth stages.
Hot temperatures also encourage bolting, which may also lead to more bitter leaves, and possibly less developed heads. Make sure you choose appropriate breeds or grow your lettuce primarily in spring and autumn. They are a short time crop anway.
That is a little warm for lettuce seed germination. A more useful number to know would be your soil temperature. There could be a variety of factors involved though. Did you keep the seed bed moist? Did you sow the seeds too deep into the soil? Hard to answer your question without more information.