You are too early to determine the sex yet. Plant height at this stage does not indicate much of anything, other than especially tall seedlings are "stretching" looking for light, possibly because they aren't getting as much as they want. If you care for the problems that they are presenting with, you should be able to save some, but there are many types of issues that can cause plant death. Photos would assist greatly in allowing us to determine the likely cause(s) of your plants deteriorating health.

What did you germinate them in, what have you added, and have you used any chemicals (it is bad to fertilize at this stage). What type of water are you using, and how often do you water. Are all of them in the same container? See the section in the FAQ concerning Sick Plant Diagnosis and Symptoms... Cannabis FAQs / FAQ , Recipes, Medical Marijuana