Ongoing problem

Hi i have been growing these plants for about 2 months now im not sure of the strain but the leaves tips turn yellow as well as the edges it seems to go away but out of three plants only one keeps doing it they receive same nutrients and same watering "when top soil starts to dry" here are some pics let me know what other info you may need im still learning what these plants need haha


Edit:
the first one i think spider mites have gotten to my plant but i have used insecticidal soap and have not had any more problems with them

the second is the one i have no idea about any info is appreciated

more details...lights? distance? humidity? CO2....need more info

Lights are 3 100 watt cfl about 10" from the plant humidity im not sure of as i dont have anything that could read that. temp is 70-75. co2 is not being added as there my first plants so i just wanted to see how they would grow with what i had before i started to invest more in to it

if your cfl's are 10 inches away get them about 8 inches closer bud, 2 inches from your plant should help show improvements, do you have a fan on them at all? and when you say you "wait for the top soil start to dry" i get the feeling you may be over watering, i usually wait till i can put my finger in the soil say to my first knuckle on my middle finger without my finger nail getting wet at all its good to water, also try picking it up and feeling how heavy or light it is. another good way to tell is if the top of your soil is pulling away from the medium, like a crack that goes in a circle around the top soil around the medium edge.
Also i shur hope your not using any fertilizer on it yet, stay clear from miracle grow. hope this helps take it easy

Yeah the soil is pretty well dry before re watering and i had a fan but do to some technical difficulties it's no longer performing (fell to the floor). u think stagnant air would be the cause? they are still in vegetative stage although i should start converting them to flowering they are getting pretty big about 27" tall