I'm not a fan of any of the MG soils and especially any with "moisture control" on the bag. I never had great success with any MG products over the years. If I had to guess it would be over watering and the roots are drowning which stalls growth. This can be caused from either over watering or dense/compact soil or a combination of both. If they start to droop but are still dark green it's also a common sign of over watering. The roots need oxygen to thrive. If not fixed fairly quickly seedlings will die from root rot. A lot of stretch there also, so lower or increase the light. Let them dry out more between waterings and add a fan if you dont' already have one to increase air movement to help evaporate soil moisture a bit faster etc.