Make sure your growing conditions are ideal so you don't stunt your plant as it is very sensitive when it is young.
Keep the humidity in seedling stage 70-80%. Temperatures around 80 degrees is ideal for young sprouts. Regular tap water at this point should be fine, but just check the pH to make sure nothing is too far off.
Sometimes, genetics plays a large factor and you are stuck with bad plants to no fault of your own.
the Root tonic could also be forcing the plant to put all of its energy into growing its roots, rather than the above ground portion. Maybe try stop watering with the root tonic and see if that helps.