As long as captionPosition is not set to NONE, then both image titles and captions will be displayed in the caption area.
There is no way to display only image titles and switch off image captions (unless you are using Flickr as a source of images, in which case make sure that both flickrShowTitle and flickrShowDescription are set to TRUE in JuiceboxBuilder-Pro's 'Customize -> Flickr' section).
If your titles are displayed but your captions are not, then it is likely that either:
(1) Your image captions are empty. You can check this on the 'Images' tab in JuiceboxBuilder-Pro. Click on a thumbnail to reveal the title and caption input fields.
... or:
(2) If your captionPosition is set to BELOW_IMAGE, BOTTOM or BELOW_THUMBS, then it is possible that the caption height is too low to accommodate both the titles and captions. Try increasing the maxCaptionHeight configuration option in JuiceboxBuilder-Pro's 'Customize -> Caption' section.
If you continue to experience difficulties, then please post the URL to your gallery so that I can take a look and help further.