Hi Padraig,
I'm agree with Andrei. By standard when you create a target group to be used in a campaign and when you execute the campaign all the BPs blockeds are not available to be contacted and standard avoid this communication.
Check if your BP has BUT000-CONTACT = 'X' or BUT000-XDELE = 'X' or BUT000-XBLCK = 'X' and change it if you need.
As Andrei says, if you need more flexibility about contact permissions you can use Marketing Permisions functionality.
Best regards.