Unable To Replace Photos For Client Download

In open orders under "manage photos" I am unable to replace about 50% of the photos to allow for client downloads. 
I have the original gallery watermarked, and have chosen to manually upload unwatermarked photos before releasing them by doing this manually under 

Open orders > Manage photos > replace photo button under the image. 

About 50% of the time I am successful, and then 50% of the time it allows me to choose a replacement image, gives me the upload bar, then does NOT replace the image even though I get no error message, no indication that it is NOT doing what is asked, just not replacing the image. 
I have even accessed the original gallery and attempted to use the button under each photo to replace and no luck there either. The only way to allow my clients to download images is to do it all through a third party site which I hate. 

What am I doing wrong? 

Are the photos that are not replacing larger than the ones that are? 

That is the only thing I can think of, possibly  being too large for the server to process.

• 1) The link / URL to your Sytist admin (where you log into your Sytist admin).
• 2) Create me an admin account by clicking Admins in the main menu, or send me your admin username and password.
• 3) Attach a photo to the email that will not replace and the order number to replace the photo on.