seedlings keep dying. Have no idea why

I seem to have no trouble germinating or sprouting, but for some reason I can't get them to live much beyond that. I have absolutely no idea why. Ive tried everything from making sure they are not too hot or cool, I make sure they have the perfect amount of water, light....
I'm just at a loss as to why my plants keep dying.

over-watering...................

without more info i would assume you germ them and then plant them too deep and they dont come up.

or they pop through the soil and fall over and die the next day. this would be called "damping off"

are you using a humidity dome? if so are you taking off the dome when they pop through the soil?

there are some plugs that can not be overwatered.
maybee that will help you?

Also, having an oscillating fan blowing a soft breeze on them helps to strengthen them up.