I second Kreuz! I'm pre-t and a hard gainer, but what took me ages to understand is that I needed a calorie surplus to gain muscle. I mean, it's not hard to understand really, but I always thought "But I'm eating enough?!" -- until I started counting calories. I don't eat meat, dairy or eggs -- fish only occasionally, because I also can't do soy and gluten (my diet is a result of a mix of food sensitivities and ethical concerns, it's a bit weird).
So for me it kind of clicked now. I try to take in about 2,000 calories (I'm 6 ft, around 66 kilos at the moment). Protein intake is between 30 and 60 grams per day, a lot of my calories come from fat (I eat coconut oil by the spoonful [emoji14]). I work out twice a week with functional exercises, weights; I swim once a week.
I have managed to gain about 3 kilos since September but without increasing my body fat (it's around 20-21%, so def in a female range and I don't want it to increase by any means). But normally what would be effective is to bulk up, gain some fat along with muscle mass, then shed the fat and look awesome

. This is much easier on t though, so it's not for me at the moment

.