What are you measuring, at which wavelength? Is there any reaction occurring inside your sample? Are you controlling temperature? Are there suspended (micro)particles precipitating or re-suspending? And what is the repeatibility of your instrument with a black/standard sample ?

Calibrate your equipment baseline correction, may be unstable dilutions may be your diluents are volaltile are temperature unstable, many reasons. I think first you calibrate your equipment.

Calibration is required. Check wavelengths of blank and samples it should be same. Your dilutions in samples could be a reason or cuvette type used for sample run clean it properly after every reading .