I've experienced the same thing... searching an old park only to come up with very little after hours except lots of trash.
I'm still new and go out with a friend who has done this far longer than I have, and he's helping to develop the appreciation. And I'm learning that just what ROBOTCOP13 said, the zen is in the hunt. I'm learning to enjoy the proverbial cat and mouse game with whatever is hiding.
I had a bad weekend last weekend. While frustrating it's a challenge to overcome. I talked about it with my mom and she pointed out that it makes you appreciate and enjoy even the small finds.