There's a few parts in play here. The hosting system, meaning fielding the survey through an account at hosting.sawtoothsoftware.com, will restrict you to 250 completed respondents if you have the Academic Lab license. When the 250th complete is reached, the Survey Closed page in the admin module will trigger and close the front door.
You should still be able to download all data manually via the admin module, which will give you a CSV file.
SSI Web, however, will limit you to 250 respondents when you accumulate your data, either by auto-downloading via the Field menu, or by importing the CSV file (it should take the first 250 completes).