I am in the same as you and lost 20 NL x5 Haze feminized costing me 380 Euros from sensiseeds.. and here is the problem i discovered.. The heat pad and the light is too much for the seedlings.. i did a germination of the seeds directly in the rockwool and they got fried (this is my third set of plants but first with rockwool).. I bought 20 more seeds, same strand and have been successful with 16 out of 20 of them coming through just fine now.. ready for transplant to the larger cubes here shortly.. the four that i lost was because of this.. The rockwool is more difficult for the young seedlings to settle thier roots than in soil.. so the seeds basically grew out of the rockwool rather ten into it.. you need to put the seeds about 3-5mm (1/8th of an inch at the most) the very moment they break the seeds.. also, be sure that you soak the rockwool prior to transplanting them. Additionaly, using about 1/4 of the amount recomended of a rooting/vegetative fertilizer and drop it at the roots with the eyedropper measurng thing everyone gets when you buy the liquid fertilizer.. also, something that i have done in the past and has been recomended to me is to put a cup of water in the bottom of the tray just to keep the rockwool moist..