Why can't I get a clear image when I try to use my 2x Barlow lens with my 6 mm eyepiece?

It’s because you’ve exceeded the practical limits of magnification for your telescope. These are determined by the laws of optics and the nature of the human eye. As a rule of thumb, the maximum usable power is equal to 50x-60x the aperture of the telescope (in inches) under ideal conditions. Powers higher than this usually give you a dim, lower contrast image. For example, the maximum power on a 60 mm telescope (2.4 in aperture) is in the range 120x-142x. The higher powers are mainly used for lunar, planetary, and binary star observations. As power increases, the sharpness and detail seen will be diminished.

The combination of the 6 mm eyepiece, 2x Barlow, and 900 mm focal length gives 300x--well beyond the maximum usable power.

The Earth's atmosphere also plays an important part in limiting the maximum magnification you can use. Instabilities in the atmosphere such as heat radiating from the ground and surrounding buildings, high altitude winds, and other weather conditions can cause your image to blur. This "bad seeing" can drastically distort your image. This also explains why bright stars appear to twinkle. The best time to use high magnification is on nights when the stars do not appear to twinkle very much.

Updated 12/27/13