could be alot of things.
Did you wash your LECA before you used it.?
is the Ph ok?
what kind of lights are you using?
is it the right light for Vegging?
how many watts id your light?
how far away is the light?
did you stress your plant some how?
Whats the air temp?
whats the water temp?
are you using 1/4 to 1/2 nutrients for seedlings?
is the Root system growing?
I could be your plant. Maybe you have a bad seed.
maybe it's just the plants genotype to grow small.
all I can say is check everything. sometimes they don't grow for shit at first
and then 2 months down the road your smallest plant is your largest.