When selecting a specific sub-site from the Content Deployment Job via "select site", you may be unable to see or select some of the subsites. They may also appear in a red or orange font. The dropdown will not appear if they are unselectable.
I was able to solve these issues by granting owner permissions on the unselectable subsites to the user I was logged into central admin as.
Alternatively, you can log into Central Admin as a user who has Site Collection Admin permissions on the Site Collection you are exporting or grant the Central Admin user Site Collection Admin permissions.
rrfreeman