Exposure problems with Pentax K-500

As a result of getting underexposed images when shooting yesterday I did some checking today and found the following.... Taking both incident and reflected light readings using my Variosix F meter and the camera meter in viewfinder mode, the result for all three was at ISO100 f8 @ 1/15... The resulting image was again grossly under exposed. Switching to Live View, the camera meter reading was ISO100 f8 @ 2.5 secs.. which gave a correctly exposed image. Can anyone please explain this as I am completely baffled.
bobmartin

One possibility is that you are set to spot metering. If this isn't used as intended exposures will be all over the place. To be absolutely sure what is happening some sample images would be helpful as it just might be a consequence of the type of subject as well.

aperture motor falure?

If you look in to the front of the lens can you see the diaphragm stopped down?