Not sure if you have been on a Princess Cruise. If you have you will be a member of their Captain's Circle. If you are a member then every P&O cruise is considered to be a Princess cruise. Therefore if you have done 5 P&O cruises you move up to Platinum Membership. Also if for example you have done a 14 night Princess cruise 70 points will be added to your Portunus Club points.You can find Captain's Circle benefits on Princess website. Not sure but I think if you have not done a Princess cruise before your P&O cruises do not count.
As far as I understand you have to do a Princess cruise first before your P&O points apply. I know some people have done a short 2 or 3 night Princess first before their longer cruise so that the points apply on their longer cruise - hope you see what I mean.