According to Smittychat, if you get redirected to the new watchlist after clicking the green Retry Import button, it is because you have already imported your old Portfolio Manager account(s) (or at least the system thinks you have) so it will not retry and redirects one to the new watchlist.
250 items are stocks or other securities in each old Portfolio Manager account one is trying to import not transactions. It does not appear to count stocks which you have sold completely and thus have quantity 0 of as they are not imported anyway. So make sure you have 250 or less with quantity and are importing them info a new uniquely named watchlist.