The detector is taking too long to respond/time constant is too slow
The first step when this issue comes up is to check the source intensity. If the intensity of the source is too high (beyond 1mW/cm2), the detector material will heat up to a point that changes the detector characteristics. This change forces the detector to take longer than it originally took to take a measurement since essentially the detector is re-sensitized which changes the photoconductive properties of the detector. The level of impact on the detector performance depends on what the ratio is of the heated/changed area to the entire detector surface area.
Source (with permission to reproduce): http://optodiode.com/resources.html