You should also check that:
1- the CRM mask and ERP mask match (format of the campaign ID)
CRM SPRO -> CRM -> Marketing -> Marketing Planning and Campaign Management ->
System Landscape -> Define Formatting Mask for Marketing Projects
ERP SPRO -> Project System -> Structures -> Operating Structures -> WBS -> Project coding mask
=> You usually have different masks for campaigns and marketing plans
2- the higher level objects (i.e. marketing plans) are released... until this is done no transfer to ERP will occur.
If the above are ok, let me know and I can give you more advanced suggestions on middleware settings to check.
Anik