Another possibility is that the Gain is set too high. The Gain sets the electrical current across the photomultiplier tube (PMT) to change the sensitivity of the PMT. A lower Gain is used in assays that have higher concentrations of a signal, while a higher Gain is used in assays that have lower concentrations of a signal. Since cells have many different molecules that can absorb light, to keep the background low it is recommended to set the Gain on the blank well to 5% of the target value. For the Omega and PHERAstar series of readers, this would give a target value of around 13,000 RFUs for the blank (5% of 260,000 total possible RFUs); while for the OPTIMA and NOVOstar series of readers, this would give a target value around 3,250 RFUs for the blank (5% of 65,000 total possible RFUs). Then all other measurements should be higher than the blank at that Gain.
For more information on setting the gain adjustment, see our Video Training Series: Optimizing Gain Setting. This video shows how to get the widest dynamic range & best sensitivity for your microplate reader measurement by gain adjustment.