Could your problem possibly be "damp off" A disease of seedlings that rots the stems at soil level. I use a product called "No Damp", as well as sterilizing the planting medium, And I also use a small fan to keep the air circulating around the pots on my table. Hope it helps.Rai

I'm actually having the same problem. I wonder if it could be "damp off" 'cause I have many many other little plants that are close to the and that are growing well.

Tell me if you find the solution... ;)

I live in the Pacific, where papaya trees grow on roadsides. These trees need large pots if you are going to grow them till they fruit. They have the largest root spread you can imagine. When their roots get cluttered, the tree will not progress. Fertilizing them will only enhance root growth which inevitably will make the plant sicker and eventually die. The yellowing of leaves is the first sign. If you try to repot the tree, you will notice the roots are crowded. These trees have a tough time growing in pots if they are to fruit. They need a lot of ground to spread. It is their nature. I suggest you try to place them in the ground. If you are unable to do this, then you will have to just get a huge pot. The biggest you can find. These trees also need a good soil mix, some clay and some sand too! Good luck.