My TurboTax home page says I should be getting back $310 on my federal tax return. Why is the check I just received in the mail only for $9?

By "...not checked for credit." do you mean that you did not check the box to indicate someone else could claim you as a dependent?  If so, that could explain the change in your expected refund.  If that's not what you meant, please explain what you did mean.  

It is not uncommon for refunds (or payments due) to be different than what was on the tax return that was filed.  This can be the result of any one of a number of things, including (but not limited to) a data entry error being made on the return, a tax document (such as a 1099-DIV, or a W-2) not being included on the return, or the taxpayer owing tax from a previous year.   Regardless of the reason, the IRS (or state) will send you a letter explaining the reason they made the adjustment.