Why does my sample with the most dense cells have the least amount of fluorescence for resazurin?

Resazurin (Cat No. 15700) is reduced by cells to the fluorescent product resorufin. Resorufin can be further reduced to a non-fluorescent product. Therefore, dense cells may have reduced resorufin to it's non-fluorescent substrate. You may need to vary cell densities or incubation time to ensure that the samples fall in the linear range of the response.