Pulse Oximeter Photodetector

I am currently designing a transmission pulse oximeter using TSL257 photodetector. However, I could not observe any PPG waveform after i inserted my finger between the LED and photodetector. It is as if the light from LED directly shines onto the photodetector and does not pass through my finger. I am using 5V power supply which is well within the operating range of this photodetector. Also, the LED I used is of 640nm wavelength, to which this photodetector is highly sensitive (according to data sheet). I tried changing the light intensity but even low intensities won't work. I have read in some other discussions that I need to produce a huge change in voltage with a small change in illumination. Can anyone please help me and suggest what I should do?