(NSFW) I can't always make my girlfriend finish?

So here’s the thing, I am 18 and my girlfriend is coming up 17. Now more often than not the sex is great and she ejaculates but only once or twice, it really gets me down because I think perhaps I’m doing something wrong. I’ve never had complaints from previous partners and she has told me that previously I have made her ejaculate lots and even reached an orgasm. I don’t think I’m doing anything different so why is it she seems to have these ‘off days’? Anyone? Answers much appreciated.