I had planted some roselle seeds in a seed tray with an old mix of cocopeat, perlite, vermiculite and compost. The seeds were soaked in water before planting for some time. Now after eight days or so they are sprouting but two of them have one seed leaf rotten. Two of them couldn't break the seed coat. I had washed the seed trays with laundry detergent since I hadn't any sterilising agent available, but it was washed off with water thoroughly. What could be the cause of this and how do I avoid it since I need to plant new ones?
Hibiscus (in this case H. sabdariffa) likes slightly acid conditions. Cocopeat (coir pith) is slightly acidic and the vermiculite and perlite are neutral, so this leaves the compost and water you are using. Compost when properly aged and mixed is also acidic but can vary somewhat depending on ingredients. So this brings us to the water you are using - sometimes water can contain various things in solution such as salts, which, though fairly innocent regarding pH, can nevertheless interfere with the functioning of root hairs and could cause seeds to germinate reluctantly. So please see what you can find out about the water you are using, and the history of what was in the potting mix previously.
Another factor is the age of the seed; generally there are few seeds that will germinate effectively after 3-5 years storage unless they were given very special treatment.