Yes I think Aqua man is correct. The roots can be really finicky either with too much water or too little it goes both ways. So in your situation it was a tiny sprout in that 1+ gallon pot and you were being really careful not to water too much but likely what happened is there was still a few little pockets of dry soil under the roots. So it was limiting the growth potential. Flood the pot good to make sure all the soil gets moist then let it go for a while without watering it should allow the roots to push into those patches that were dry before.