I'm betting it is the temp, add a some type of heat and see if they improve.

I have not planted any seeds yet I always wait until after March 20 for my plantings. Not only doe you need to concern room or hot bed temp but also outside soil temp. May 15 to June 15 is perfect time to plant in garden.

Plant a tomato or pepper on May 1st and another of same variety in late May, both will produce fruit at same time. It is all about soil temps.