I'm relatively new as well. It's hard to do, but when your GPS says 15 feet accuracy, and your within 15 feet, put your GPS away, and just start looking for out of place things.
I find myself pulling up to a pine tree thinking "oh, I'm looking for a film container" when in reality it's a bison tube, or a locknlock. Try not to out think the cache.
The other thing to keep in mind, is the accuracy of your GPS is only as good as the accuracy of the hider. What I mean by that is some people have different GPSr than you, and they are more or less accurate than yours.
Take a look at the logs as well, if you see 4 or 5 DNFs in a row, then the cache is most likely missing, but there are exceptions.
Good Luck and Keep Trying