The ratio of plant size to room size is one of the biggest factors with humidity closely followed by the amount of ventilation. This is where you should look when wanting to change your humidity.
I don't worry about low humidity too much. I often find my humidity is low during early veg and it is one of the signs to me that it is time to switch to flower when daytime humidity rises above nighttime humidity because they have reached the right size for their space and won't end up too big.
If they are still small then you could reduce the amount of air you are exchanging. This would let them build up the humidity in the room. Watch your temperatures though, if it gets too hot turn off your light for a bit and it will get cooler and more humid. Your plants will like that and they won't notice an hour in the dark.
If they seem happy they probably are. The humidity will rise as they get bigger.