I believe that jiffy pot is retaining to much water and not allowing proper oxygen into ur roots. Which is causing ur yellowing leaves. Solution let the medium dry 3/4 the way than get it transplanted into a larger medium, with better drainage. And have a better wet to dry cycle for your root system to function.
Do you have any drain holes in the bottoms of those transplant cups? How much are you watering and are you using a Supersoil with added nutes? Could be a slight cause of the soil being a little hot for it starting out or overwatering. Cheers and happy growing!