For a PKG to be installed in a Configuration using Casper Imaging, you want to make sure that "Install on boot drive after imaging" is enabled for each of the packages. This can be set in Casper Admin, select the package and verify the settings on the Options tab.

If it is, then you'll want to review two important log files to learn why they failed. Open the Console application on the recently imaged Mac. Spin down to the reveal the logs under /var/log and look at the logs for "install.log" and "jamf.log"

The jamf.log will show the order items were installed and might indicate if it was Successful or not.

The installer.log may offer greater details about success and failures. While reviewing installer.log, press 'command'+'f' to search on either "fail" or "error". Cycle through each to see which package failed and the reason why.

@winningham.2, I've found that most "direct from their maker" .pkg files must be set to "install on boot drive after imaging" otherwise they fail to install. If I use Composer to create my own .pkg file then sometimes I can have it installed prior to first boot. It depends where the stuff in the .pkg file needs to be installed. In my experience, if the .pkg file needs to install anything in a users home directory then it has to be done after imaging.

I set every single package to "install on boot drive after imaging" just to be safe. Some packages can't install properly to a target drive.

Thank you to @mpermann @jhalvorson for your quick replies. I was unaware that option existed, perhaps because I always used to use DMGs until recently. Who knows... But that option was the trick and I greatly appreciate the help.
Cheers!