They are growing slow as you say...i would suggest you to rise a bit the temp expecialy during night time ( roughly 19°C)...and double check if they are in an under/over watering situation...are you giving them some nutrient feed?
@jackjack, Ok i'll see what I can do with the temperature at night.
I don't believe I'm watering badly but watering and nutrients seem to be an issue for me that I can never get right, so I'm abstaining from nutrients at the moment.