Seedling stall

So.... I get the seeds germinating and all popped in a couple, three days, as usual... then put them into soaked rockwool cubes (pH 6.2), as usual...then set them in a tray under a dome with a pair of T5's, as usual... and they sprout, as usual...

Then they totally stalled.

Not sure what I'm doing wrong, as this all worked well before. I wondered if it was that the root zone temp was too low so I got a heat mat and thermometer. It doesn't work well. It gets the root zone up to 75-77f but then drops way down to 70/71f.

The seedlings in the pics are 8 days from the start of germination. Usually by now the roots are showing through the bottoms of the cubes. Today is day 10 and still no roots showing. Could it be that the heat mat is throwing too much heat up at the bottoms of the cubes? (I have the heat mat sitting inside the tray directly underneath the cubes, which are suspended above the mat by an inch, because if it is under the tray the heat mat can't warm through it enough to get the root zone temp up to 7f. Also, I have to keep the dome on for the same reason.)

Maybe I'm over-watering???? I dunno...how much water should I ad once the cubes start to dry out?

Any ideas?

I would think that the heating mat may need to make contact for it to be effective. I have my heating mat right under my drip tray, then cubes in containers rest on drip tray.

You're over thinking things. It's a weed.

Also get some more light on em unless you're growing spaghetti sprouts for a tasty sandwich.

Nawww... it's the strain. White Russian is lanky. Works fantastic for scrog.

Well...I don't know what I did wrong. Maybe it was over-watering or maybe it was too cold of a root zone. Whatever the case, by the time I got the heat mat under the cubes it was too late. Over the next week the sprouts did nothing. So Killed them all.

And I started again. But I ran out of White Russian seeds. Instead I started germinating 4 Romulan diesel seeds and 9 Black Ice seeds. I got the BI from HGS a while back. I never heard of the strain but it is supposed to be indica dominant. According to their website it is a cross of Black Queen (White Widow X Zombie Black) X Cinderella 99 (Jack Herer X Shiva Skunk).

Anyway, 3 of the 4 RDs popped and all of the BIs popped. Here they are at 8 days from germ. Decided not to bother with cubes anymore. While they may be more convenient I have had trouble with seedlings stalling. Just hjad better success and faster grow rates with beer cups.