Unfortunately, IPTC data entered using IrfanView does not seem to be picked up by JuiceboxBuilder-Pro.
If you enter your IPTC data using an Adobe program (such as Photoshop or Lightroom), then there should be no such problems.
For example, if you use Adobe Photoshop to enter your IPTC data, then use the IPTC Document Title field for the image title and the IPTC Description field for the image caption.
As far as I am aware, IrfanView does not write the IPTC data to an XMP block like Adobe applications do.
JuicboxBuilder-Pro, which runs on an Adobe platform (Adobe AIR), looks for IPTC data in an XMP block where other Adobe applications would write the data to.
I realise that not everyone has access to Adobe products but you might be able to find another free image editor which allows you to enter IPTC data in the same way that Adobe products do.
In fact, if you are a Windows user, you can just right-click an image, select 'Properties', go to the 'Details' tab and enter a 'Title' in the 'Description' section. This should be picked up by JuiceboxBuilder-Pro (in both the title and caption section) and might be suitable for your scenario.