I find temperature to be VERY important. Last year I put the container i was using to germinate into my clone chamber, on the seedling mats. The six plants I had in there with the seeds must’ve hogging the warmth because I had 3 batches of seeds fail. I went back to the way I was doing it before I made this clone box. The fourth batch of seeds went on an old heating pad (pre-automatic shutoff). 12 hours all 10 seeds had popped. I didn’t even need to pre-crack the shells. The heating pad gets to about 87° on low. Obviously, a heating pad with auto shutoff will NOT do the job.
I keep the mats in the clone box to 82.5° with a thermostat. At that temperature the first three batches had been in for 3-4 days and instead of sprouting, they got moldy from sitting in water for so long. Generally speaking, I toss any seed that hasn’t sprouted in 48 hours of germinating. Twice a year, I make a plant full of seeds for each of the strains I grow. I usually get several hundred per plant. I only maintain an inventory of seeds as a “just in case”. My latest battle has been with thrips and those little flyin’ fuckers are a vicious adversary. I’ve needed to germ seeds for 4 crops in the last year alone because of these damn fuckers taking out grown plants. I believe i finally have them (nasty flyin’ fuckers) under control, but it has been taking an order of 25M sf nematodes every month. 25M nematodes weighs about 20 grams. The 1st application all nematodes are very lively, so I use only 4-4.5 grams of nematodes to .75 gallons of water and split it up for all my pot plants AND the houseplants. Doesn’t do to wipe them out of my grow rooms only to let them thrive on my 30ish houseplants. 
BTW, when needed, I use a pair of hemostats to crack the shells. I find I have better control on how much pressure I apply. I had tried tweezers, but I ended up destroying most of the seeds. I never put any kind of blade to my seeds.