If you can't move the stage up to the focal point at 10x.The refocus stop on the oposite side of the tension adjustment side may be locked in place or the friction plates may be sticking. The tension adjustment on the other side maybe to tight. Please do not turn the coarse focus knobs aginst each other- doing so will break things. The e400 is out of production- Nikon still has a few parts.
By clamp knob do you mean the tension adjustment ring that is located on one sde between coarse focus and microscope body. It works by increasing friction. Hold the coarse focus knob and try to turn the tension ring. Some scopes have locking set screws in tension ring so you may need to loosen them slightly with correct sized metric driver.