Only listings with an exclusive buyers agency agreement can be added to Loopnet, so if you've checked that it's co-marketed and have uploaded a co-marketing agreement, that will be the reason why it isn't on Loopnet.
However, if that does not apply to your situation, it may be an error. In this case, please submit a ticket via the Guide with the Listing ID and Property Name and let us know it's on Crexi, not Loopnet, and that there isn't a co-market agreement, and we will investigate.