Dichondra silver falls

I sowed around 30 seeds of this almost a month ago, and only 3 seedlings have emerged. I am so disappointed. Are they normally hard to grow, or have I done something wrong?

 

Windowsill propagator, with a sprinkle of vermiculite on the top as per the instructions. Seeds were from T&M though, so am starting to wonder if I am getting the same as I had with their seeds last year. None of the non-stop begonia petticoat that I sowed at the same time have germinated either - not one. The begonia boliviensis have germinated though - all the begonias are in the grow cupboard with heat and tons of light.