If you use a mix with peat moss in it, this is especially important. Perhaps the seeds are planted a little too deep. Small seeds usually need only be pressed into the surface. Larger seeds are generally planted at a depth 3 times their narrowest width. I generally use a light sprinkling of vermiculite on top.
Seeds have a finite amount of energy stored.
If planted too deep, they will use all that energy just trying to reach the surface. If they don't get there, they can't manufacture their own food. The stored energy is gone. It should be moist, not swampy. The fluctuating temperatures you mentioned could be the problem.