I have just tried converting a SimpleViewer (v2.3.1) gallery to a Juicebox gallery using JuiceboxBuilder-Pro (v1.3.3) and it seems to work OK for me.
The image captions from the SimpleViewer gallery are translated to the image captions in the Juicebox gallery.
(The image titles in the Juicebox gallery are blank because SimpleViewer does not support image titles.)
Please check your converted gallery's 'config.xml' file (open it in a plain text editor) to see if the image captions are really there or not. If you like, you could post the URL to your gallery so that I could take a look and help further.
Better still, if you are willing, you could zip your original SimpleViewer gallery and upload it somewhere for me to download so that I could try the process for myself with your own gallery and determine exactly what the cause of the problem is.
It may also help to know what version of SimpleViewer your original galleries are.
Thank you.