Let's start at the top and check off each item:

Is the flybar bent? Does it bob up and down freely?
Are the top blades cracked, chipped or otherwise broken?
Is there any damage to the linkage between the flybar and the top blades?
Is the Inner Shaft bent?
Is the outer shaft bent?
Is the lower head installed correctly? Are the top screws going through the holes in the outer shaft?
Is the outer shaft binding on the swash? Is the swashplate installed correctly? Are the linkages damaged?
Is the retaining colar tight against the top of the upper ball bearing? Is there any play in moving the outer shaft up and down (it should not be able to move up or down)
Are the teeth on the main gears broken or stripped?
Are the main gears meshing properly with the motor pinions
Do the upper and lower blade assemblies spin with relative ease? Are they binding?
Are you sure your battery is properly charged?