The more I think about this the more I think it is the CG.

Where do you have the plane balanced? Tell me relative to the molded marks, or from the front of the wing where it joins the fuse.

Are the batteries you are using flight batteries or is this a pack that was intended to go in a radio? It is possible that these are low performance batteries and can not sustain the current requirements needed by the motor. You need to provide between 10 and 15 amps.

When you hit he throttle, does the motor sound like it is running at full speed? I tried running a speed 600 on my Aerobird packs. It ran, and the motor sounded OK, but when I put two in parallel so I could get more amperage, the motor really spun up. That needs to run on sub C type cells or perhaps the KHN packs.

So, in order of importance:

Check and double check the CG - Set it about 8 1/2 inches behind the point where the wing meets the fuse. This is about 1/2" forward of the molded marks which seem to be for a very aerobatic rear heavy flight plan, not for stability. Mine would fly there, but I could not control it!

Make sure your battery pack is giving full voltage 9.6V+ After being fully charged and after it has been allowed to cool down. It really should be closer to 9.8-10V.

Make sure it is intended as a flight pack!

Borrow batteries from someone else to try. Don't even fly the plane, just see how the motor sounds.

Check your elevons to see that they are adjusted even with the wing. It is a little hard to judge on this plane. You may have too much down adjusted in.

Have you tried adjusting with the trims on your radio for up elevator on launch?

Does the plane go 50 feet and crash, or does it go 50 feet and just glide into a smooth landing even though the motor is running?