Start by checking if your financial institution or brokerage is a TurboTax 1099 partner (opens new page; scroll down and click the 1099/1098 Partner tab). If your institution isn't on the list, you won't be able to import.
Also make sure you're selecting the correct partner in TurboTax. Some have similar-sounding names but are in fact separate entities. For example, J.P. Morgan Chase Bank, J.P. Morgan Funds, J.P. Morgan Private Bank, and J.P. Morgan Securities are all listed as TurboTax partners, and if you choose the wrong J.P. Morgan, you won't be able to import.
Once you've confirmed that your institution is a participating partner, make sure you're using the proper login on the import page and that you have a working internet connection. A wired connection, if available, may be more stable than wireless.
Still having trouble? Bear in mind that even if you have the hard copy of your 1099-B in hand, your institution might not have it ready for import until late February or early March, sometimes even later. We'll display your institution's "ready for import" date in TurboTax. More info.
Finally, if you're still not able to import your 1099, you can always enter it manually. Select I'll type it in myself or Skip Import and proceed.