Resolution Issue

I am creating using CS4 a book that photos, 76 to be exact.

ALL 76 CAN look great when the resolution is set at High Quality Performance.

However, I have found that 2 of the 76 have the mysterious quality of looking very pixelated, or. at other times, can appear as sharp, just like the other 74, while having the resolution set at High Quality Performance.

I have no idea why this is the case and explored every possibility I can think of. The problem is strictly affecting those two photos.

Any help would be greatly appreciated.

MFJ