I make my Gf finish but I cant??

Just seeking some advice about this topic.. I (M22) have started seeing this girl (F22) and the sex is great, feels amazing but I cant finish?

Always start with plenty of foreplay and make my way down on her which she loves, then proceed to do the deed which she is great, feels amazing but she always finishes before me and have never had this problem which is kind of throwing me off I think, any advice on this subject would be great