Both GridRepublic and BOINC projects identify users by email address. So when you register for GridRepublic, it is important that you use the same email address as you used previously for registering at your BOINC projects.
If you do use the same email, then GridRepublic will automatically find and "import" your projects' account information upon registration. (You can stop using GridRepublic at any time; joining or leaving GridRepublic has no effect on your project accounts, credits, etc.)
If you used a different address when registering at GridRepublic, then the system will not find your preexisting project account and will conclude that it is your intention to create a new account at the selected project. (And of course this new account will have zero credits).
Thus the simplest solution to this issue is to create a new account at GridRepublic, this time using the same email as your BOINC accounts.
NOTE: if you are participating in World Community Grid, then your GridRepublic "Name" must match your World Community Grid "Member Name". (World Community Grid identifies users by Name rather than Email.)