Pixelation in Resized Images

For the first time, I have encountered a problem after resizing a jpg in Photoshop.

I   need to reduce the resolution to 72 and set the dimension to 900 pixels on the longer side.  One set of images -- all shot with the same lens -- appears pixelated after resizing.  This occurs when I've started with a TIFF or PSD.

Has anyone else experienced that?