I increase RH with a humidifier that I purchased at walgreen's. You can do it to a lesser extent with a cloth dipped in water so it wicks the water and a fan blowing on the cloth.
If you are going half notes (and you aren't using miracle grow soil or something elseof the like with all kinds of nasty fertizilers in there), then it's probably not nute burn. You might need a bit of nutrient deficiency - try some cal mag or a teaspoon of epsom's salt in a gallon water.
Sounds like you may just need to water a little more often. I check the soil by putting a finger into the soil up to the second knuckle. If it's damp, no water, if not then get out the watering can.
Just hang in there and keep working it