The reason why the caption area does not scroll when the gallery is viewed on mobile devices may be due to the fact that when the gallery is displayed in Small Screen Mode, the caption area is always overlaid on top the image.
There are already touch gestures associated with the gallery (tapping to toggle captions on and off and swiping to navigate between images) so these might somehow be interfering with the scrolling action.
Also, there may be issues with stacking (making sure that the caption area is stacked on top of all gallery elements, including navigation hit areas, so that the container is scrollable).
Unfortunately, I've not found a solution which works on mobile devices (I've tried a lot of CSS on a lot of different classes without success) and I expect that it might be quite difficult to implement (which might be why Juicebox-Pro was not designed with a scrollable caption area).
The only thing I can suggest is more of a workaround.
If your text is only just being truncated by a small amount, then you might like to consider reducing the font size of your image titles and/or captions so that more text fits into the caption area.
There are no configuration options available within JuiceboxBuilder-Pro to change the font size of image titles and captions but you can style individual image titles and captions (for example, to change the font size) using HTML formatting as noted in this FAQ:
How do I add HTML formatting to image captions and titles?
A sample caption with a font size of 12px (for example) would look like this:
<span style="font-size: 12px;">Image caption text goes here.</span>
In the gallery's XML configuration file, the caption would look like this:
<caption><![CDATA[<span style="font-size: 12px;">Image caption text goes here.</span>]]></caption>
... but JuiceboxBuilder-Pro will automatically add the <caption> and CDATA tags so you only need to enter the <span> tag (as above) into JuiceboxBuilder-Pro.
(You can do likewise for image titles.)
Otherwise, you can set the size of all image titles and/or captions at once using CSS such as the following (changing the numeric values as appropriate):
/* IMAGE TITLE */
.jb-caption .jb-caption-title {
font-size: 20px !important;
}
/* IMAGE CAPTION */
.jb-caption .jb-caption-desc {
font-size: 18px !important;
}
/* IMAGE NUMBER */
.jb-cap-frame .jbac-number {
font-size: 12px !important;
}
As with my previous CSS suggestion, you can add this CSS to the end of your gallery's 'jbcore/classic/theme.css' file or wrap it in <style> ... </style> tags and add it to your gallery web page's <head> section.
I realise that this does not directly resolve your problem but I hope that it is a suitable compromise.