You'll need to share:
a) The details of your "lock expression" used in Manage | Security
b) the design of your layout. A portal means that you have a relationship and possibly a portal filter. What exactly have you set up in terms of your portal and the layout for this portal?
Then we can compare your set up in Manage | Security to your design for this part of your database to see if we can spot any possible issues responsible for the result that you are getting.