Lock screen Photos not showing all

[wagga-ria]

Hi All

I have a Surface RT which I used as a Digital Photo Frame.
I had about a couple of hundred photos in several sub-directories in the SDXC card.

When I lock screen the RT, it starts to do the random photo scrolls, which is the intent.
After a while however, I do notice some photos get to display more often then the rest and some don't get shown at all.

What do you reckon could be the issue?
It did display "random" photos from different sub-directories. Just that some photos will never appear during the scrolling.

Thanks in advance