check the FITS header of one of the images (open it onto the desktop in PI, then select File > Fits Header.)
if the IMAGETYP keyword is not present in the file generated by Sharpcap Pro, that is the problem. chances are it's missing a bunch of stuff because that keyword is pretty fundamental.
you can still add your frames using "add custom..." - in that dialog there are drop-down menus for setting the filter name, frame type and duration.
rob