This happens occasionally. Close TurboTax, then try clearing your browser's cache and deleting your cookies -- this will sometimes facilitate a complete import. See the TurboTax Help articles How to clear your cache and How do I delete cookies? for guidance.
Then, make sure you're only downloading information from one brokerage at a time.
If these suggestions aren't helpful, see the following TurboTax Help articles for potential fixes:
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