I have been searching for one hour and there still is no Ralts! Why?
Ralts is found on Route 102. It has=s a 4% encounter rate when you walk around, and a 5% encounter rate when you encounter a hoard. Try using the Dexnav, it's a lot easier to find one that way.
Ralts is a rare Pokemon, so it might be because you just haven't been lucky enough to find it yet.
If you're in AS you might want to try to use DexNav.
Hope I helped!