The causes for these kind of results can be several, the most common are the following ones:
DNA Template concentration is too low.
Important Note: measuring DNA concentration by UV absorption is often inaccurate and concentration is frequently overestimated. Agarose gels or espectrofluotometry are a much better method to estimate quality and quantity of DNA samples. In this case the Raw Data signal is low, around 100, due to poor DNA sequencing reaction.
No correct primer was added to the reaction, so binding between the DNA and the primer cannot occur. No DNA sequencing reaction, and hence Raw Data signal usually below 100 units.
Template purification is not working properly. Sometimes the sample preparation is contaminated with ethanol or other molecules that inhibit the DNA Polymerase, yielding no sequencing reaction.