calgoddard got it exactly right about why it flys nice close to the ground. Ground effect improves the lift of any wing.
As to not climbing, the usual suspects are too heavy, motor not wound HARD enough, too high prop pitch (or too skinny motor), not enough elevation from the rudder.
Oh, and for nice cruise in mid flight, you want the plane cg and elevator trimmed VERY close to a stall, just short of it. In fact as you descend on low power you may see some stalling. If so, you are close, trim for just a hair less elevator at that point. That should give you a good climb at the beginning with a FULLY wound motor. And yes I know about backing off to avoid ramming into the ceiling. You have to get TO the ceiling first.
IF everything else is right, (minimum weight, max wing, trim seems right, motors wound to near breaking, right prop motor combo) and your plane just won't climb, try adding a tweak of wash-in to the inside (usually left) wing. Washin means you raise the leading edge SLIGHTLY wrt to the trailing edge at the tip vs at the root of the wing. Said another way, the angle of attack for the inside wing tip is higher than the angle of attack for the root or outside wing tip. In still other words, if the wing is level to the motor stick at the root rib (center of wing), the leading edge should be say 1/16 to 1/8 inch higher than the trailing edge at the tip. Go slow here. You are trying to make the plane fly level in the circle, maybe SLIGHTLY inside wing low. Note, the outer wing should remain flat, same angle of attack at root and tip. I'm not really sure why this works, but it does, repeatably.