I got the "I Have a Boyfriend" answer today

So I was in the mall today with one of my friends looking around and I stopped by this one shop to look at some jackets and I noticed this cute girl working there. She said "How are you doing today?" as I walked in and I said I'm doing good and my friend tried to help me out and strike up a conversation with her and I got her name and I told her I was looking for a jacket for winter time and she helped me pick one out and we talked and as I was buying the jacket I asked her "Maybe we can go out sometime?" (because I wasn't sure of the next time I'd see her) and she shrugged and said "I have a guy....but maybe someday" so I said ok and shook her hand and said it was nice meeting her and I continued on. Im starting to get frustrated because it seems like whenever I ask a girl out they never want to for one reason or another, Im just trying to figure out what I am doing wrong.
Any suggestions?

Well if she had a boyfriend then it wasn't anything you did wrong O_O

Really, would you have liked it if she had agreed to go on a date with you and you found out later that she was taken? lol

Yea thats true.....I just know some girls will say that they have a boyfriend so you'll leave them alone.

But she's cute? Cute girls are generally taken? :o

And if it is something you're doing (idk if it is coz I agree with Ariadne that you can't help it if she's taken) then keep trying! It's better to try, fail and try again than try, fail and lose.

I'm sure you'll find the right one one day :)

For one, she's working there, she's meant to act really friendly regardless of whether she actually likes you or not. Not the best basis to gauge actual interest.