The Combro is a good tool, it is however very critical that it is aligned perfectly to give accurate results. Imagine the 2 light beams going across the pellets path, if one is broken by the top of the dome and the other by further down the side of the dome the reading will not be the true velocity. With the sensors only 50mm apart a .5mm error equates to 1% or about 8fps. Less that perfect alignment only triggering one sensor could be your problem? Fluorescent lights can also upset them.
One of the ones I had needed packing under just one end of the mounting to get both sensors to the same height and mounting on a silencer always gave more dependable readings. A fat drinking straw slid into the silencer can help you visually align the Combro.
HFT, for people with only 2 buttucks!
VooDoo TwoTwo The next chapter.