When using the Render button in the cart pane, larger products like book spreads with multiple images or wide format items may fail to produce flattened images.
This is almost always a memory allocation issue. If you are in 32 bit Windows such as XP, able to overcome this issue easily as the maximum memory is not very high.
If you are in 64 bit Windows, you may be running the older installation in Java that is not using a very high memory allocation. Remove the current desktop icon and visit the lab page for the ROES Client, which should give you the ability to use the native installer via the launch.msi file.
Once it finishes and opens, this should then allow you to return to the saved order and render the items.