First of all, please make sure that you are using the current version of Juicebox-Pro (v1.2.0).
Instructions for Upgrading Juicebox can be found here.

Ordinarily, the caption text should be vertically centered in the caption area so make sure that your web page contains no generalized CSS rules which the gallery elements may inherit. Apply CSS rules to only those elements on your web page which require them through use of ids and classes.

Adding a couple of line breaks (<br><br>) to the end of your caption text may also help push it up the caption area.