There are actually a few reasons for this...
It could be a mixing issue. No matter how clean your samples are, more than likely there will be another sound clashing with it and will make the end result sound terrible. If that's the problem, then you will need to need to become familiar with your Equalizer and Compressor soon. I'd explain how to EQ your drums to make them sound better here, but that's a whole course by itself. I suggest searching the forums for that topic.
Another issue could be you just have really crappy quality samples. That might not be it, but if it is, try searching on google for some nice drumkits because theyre out there and it really makes a difference.
And my last guess is that it could be that no matter how good your samples are, your drum pattern may just suck. If that's the issue then that can be fixed. You could just research how to stank up your drum patterns and make them sound more interesting. I'll tell you right now that adding swing and varying the velocity and panning of your drums are three of best and easiest ways to make your drums sound better and more natural. But again, that's a course by itself.
I'm not sure if I even answered your question or not, but those were my only guesses. You might actually make some impeccable drum patterns but I wouldnt' know since I've never heard your music before.