MacBook Pro Aperture to Photoshop photo (limit

Hi, please help me out.

I use Aperture as my workflow program for photos and do special editing in Photoshop from here. When I use Photoshop CS6 as my External Editor from Aperture 3.6, not all the photos selected in Aperture will open up in PS. For example: I want to edit 10-15 RAW photos in PS; I select all 10-15 photos in Aperture and hit External Editor Button, but only 4-6 photos will open up in PS. I've never had this problem on any of my older Macs. Why is this happening? I use a MacBook Pro (Retina, 15-inch, Mid 2014) with OS X Yosemite 10.10.2; 2,2 GHz Intel Core i7; 16 GB 1600 MHz DDR3 with graphics Intel Iris Pro 1536 MB.

It has done so from day 1 when I started using it. Any settings that needs to be changed somewhere? Or is this a hardware problem (which I doubt). Thanks.