Thank you for your prompt response Steven - much appreciated.
Perhaps JuiceboxBuilder-Pro is failing to add a particular image.
If this is the case, then try adding your images in small batches and then one at a time to try to find the image causing the problem.
This appears to be the culprit. Not sure why yet. It does not matter what set of images I try to load there seems to be a failure on load on certain images with no immediate obvious reason. Still trying to determine what is causing it.