Too bad about the seeds. Maybe you can get replacements?
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Question... you said that they're on top of the cable box... You're not leaving them up there AFTER the root pops out are you? That heat might be what is killing them. It get's almost 100*F on top of those things. 100*F Soil temp will shrivel young roots faster than you can say "Oh crap!"
Germination mats get to 80*-85* max... Just set the cups you're germinating in in a window where they'll get morning sun, or evening sun. I bet you'll have better success that 2/6.