Answer: You may be putting it upside down. The "up" side should have a larger scar where the root emerges from the end. It may also be good to wash the potato in a 10% bleach solution prior to setting it in the water. Make sure the water is fresh and the jar dishwasher clean. Such steps are usually not necessary, but since you are having problems may be an extra precaution.
Also, put the potato in a warm area. If they are cold they will just sit there and not want to grow. Such an environment is likely to result in decay problems.
Good luck with your project!
Q&A Library Searching Tips