Wow, thank you @swolny @esoth - was not expecting so many responses. Thank you all for your advice.
I tried attaching the DEM and the vector shapefiles. To answer your questions, these do overlap. Have not had luck uploading on this forum (perhaps due to file size of the raster) so will try to email to esoth@stanford.edu.
Perhaps there has been an issue with the projection CRS? Or something along these lines? Çould this affect the results?
Thanks again.