Please measure the OD with a spectrophotometer at 260 nm and 280 nm. The OD 260 values should be in the range of 0.05 to 0.8 to give reproducible and reliable results. The OD 260/280 ratio should be about 1.8. Values lower than 1.6 and higher than 2.0 indicate contaminants in the sample that interfere with the determination of the concentration and might inhibit the sequencing reaction.
If possible, we recommend measuring the ODs at 230 nm and 320 nm, too. A high value at OD 320 indicates a contaminant. This should be ideally 0.0. The OD 230/260 ratio should be lower than 0.6.
We also recommend ensuring the quality of your DNA by running your sample on an agarose gel.