Check your CRAP, in case you didn't know, means...
C: Control surfaces - Test your controls and make sure everything is secure. Ensure linkages are connected and won’t fall off.
R: Rips and Tairs - Examine the body of your plane closely looking for rips, tears or crinkles.
A: Angles - Look for structural weaknesses. Eg. check the angle of your tail and make sure it’s not crooked.
P: Power - Check all electronic connections and make sure everything works correctly.
Is your prop spinning independently of the motor when it's on. If you hold on to the motor the prop SHOULD NOT MOVE ON IT'S OWN. Sounds obvious, but a prop that can slip is a prop not at full power.
With your plane off, turn on the radio and max the throttle. Then plug the plane in (best to do this with the prop off, btw). The motor should NOT spin up. Instead it should beep funny at you. Move the throttle all the way down. It should beep funny again. When all this beeping is done you will have tuned your ESC to the limits of your throttle. Theoretically the ESC should come this way, but sometimes they don't. Not having full power could be your issue.
But what luvmy40 says holds true too. load it up to fly, make sure it balances where it should, then toss it POWER OFF into some bushes or something else soft. If should glide gently to the ground, not nose dive, not go up. If it doesn't glide, something is off.
A video or extra photos could help. A side view maybe. Kinda looks like you have some down elevator from the photo you posted, but I cannot tell for sure from that angle, so don't quote me on that.