I'd be a bit concerned about them getting too wet, the soil needs to allow air in to the roots. You are unlikely to be over nuting them. Fresh air to the leaves is important as this provides raw materials for growth as well. I guess lights are ok, temps are not too bad, nutes seem ok, check for over watering / flushing. Soil is pretty good at buffering ph problems. Perlite / vermiculite is a pretty cheap additive and helps with drainage, these plants like a lot of air in the roots. Maybe mix some in the next time you increase pot size. If it is easy for you to get some root stimulant eg Canna Rhizotonic £10 250ml, this will help young plants get established and stable. Try a leaf spray with some carbonated water and 1/2 or 1/4 nutes. This should show a quick improvement or deterioration ie over 24hr. If it makes things worse just wash off with more water spray.