Take a look at our preflight checklist.
I help a lot of newer local flyers trim in planes. Most of the time they are having trouble it is not just one but a combination of these things.
1. Their CG is too far back. Flying wings won't fly tail heavy... ever. (add some weight to the nose and try the plane with the CG forward)
2. They have too much movement it the elevons. (This shows up as a snap roll on take off)
3. They have poor leverage from the servo arm to the elevon horn. (This is usually a higher speed problem and the plane won't pull out of a dive.)
4. They don't know how to launch the plane. (Don't spin it at all when you launch.)
5. They have a plane above their skill level.
6. They are flying with their thumbs and not grasping the stick with thumb and finger. Some planes are so responsive you need two points of reference to have full control.
7. I have had several flyers that couldn't fly with the expo on. I recommend turning Expo off.
8. .... and an occasional prop on backwards. I did this myself this year!!! (writing on the prop should face forward)