Tips on Seedlings?

I have been growing for a while now but only from clones. I am having incredible success with DJ Shorts Blueberry and SFV OG Kush. I just got some seeds of Serious' BubbleGum. I have NEVER grown from seed before! I planted 12 in jiffy pods in a little humidome on top of a heat mat (same mat Ive been using for clones). Only 4 of them popped. I know how to sex them and what to do from that point on. Im looking for tips on how to make these little babies strong and healthy. Also... why the hell did only four of these pop? I got 2 packets just in case this might happen. Any tips on sprouting these other seeds or how to care for seedlings would be greatly appreciated. Its kind of embarrassing that I don't remember what they taught me in 4th grade about plants reproductive cycles so please don't be too harsh on me lol.

IMO the following are root causes of poor germination:
1. Planting too deeply is a major cause and number one cause. Seeds only need to be buried a 1/4" n no more.
2. Cold soil is another reason for poor seed germination (but ur using a dome n heating pad to control enviroment so shouldn't be a problem)
3. Third factor is over watering. Seeds need to be moist but not damp. Use the dunk method to keep the cube moist and prevent dampening disease.
4. Make sure ur using RO water with no nutrients

Many here on RIU may refute utilizing dome n heat pad but I will completely disagree with those, as seedlings are most vulnerable and like a very controlled n stable enviroment though it is possible to have good success without but why risk it.

Ok I am DEFINITELY over-watering them!