I think we may be a little confused by some of your terminology. We generally refer to 2-barrel pens (particularly rollerballs & fountain pens) as the upper (cap) barrel and the lower (nib) barrel and list the bushings in order from the top end down to the nib bushing. It also helps to post photos to show exactly where your problem lies.
There are two other issues here - there is a Jr Gent and a Jr Gent II in RB/FP. It isn't clear which you have. Also, the Kojent is available in both rollerball and ballpoint form which use different bushings.
If you have Kojent rollerball bushings, they will work on Atrax, Triton, Jr Gent II and most, if not all Jr II kits. Jr Gent II is available in post able & non-postable versions.
If you accidentally used a Kojent ballpoint bushing, I think you are out of luck because I don't think any RB has those dimensions.
The main lesson to learn is to always check bushing dimensions to actual kit dimensions with a digital caliper before turning to be absolutely sure you are using the right bushings & also because there are often slight variations from one set of bushings to another and from one kit to another.