Are the two photos in question relatively large (i.e., larger than 8000 pixels in either dimensions)? In that case, this may be due to the texture size limit of your GPU (we're already tracking this issue, but as a workaround you could try to shrink the photo).
If this is not the case, could you try opening the photos again, like in the screenshot above and, when the exclamation marks appear, select
Help -> Show log files from the menu and post the
project.log file here? This might help us understand what's going wrong.
(If it helps, the Tropy teams also speaks German, French and Russian.)