Check IIS/WebDAV Configured Correctly
Check Primary Server is a member of the local Administrators Group on the Secondary Site Server
Check Secondary Site Server has been added to the Primary Site Servers Group SMS_SiteToSiteConnection_<SiteCode>
Check in SCCM Administrator Console Primary Site Settings that Addresses have been configured for the Site
Have you seen any info in the sender.log about it communicating with the primary?
Rebooting the server might also help, i have seen it sit on pending for hours doing nothing but after the reboot everything synchs up