Problems Getting My Girlfriend Wet.?

Most of the time that we are about to have , its her idea. She always asks me if i want to, and she always seems real into it when we are starting off. But then, i always have to use a lubricant because she has trouble getting wet. She is obviously down for it, i mean, you can see it, and its always her who starts. Were both confused as to why it wouldnt. Were both young, it shouldnt be happening. We are together asking this right now, what could it be?