About how much short?

Are you due both Federal and State refunds?   Did you check to see that it was indeed the Federal?  Was it from the US Treasury?

Be sure the amount you were expecting does not include the state refund amount.  Do not go by the total refund amount.   Go by the Federal refund amount shown on your Form 1040, Line 20a.

Did you have any TurboTax fees that you chose to pay out of the Federal refund?  If so, there is also a 39.99 service fee (44.99 for California filers.)    With that option, the refund comes from a third-party company, not the IRS/Treasury.

If the IRS reduced the amount of your Federal refund, then you should get a letter from them in about 3 weeks explaining why.

If the total amount of your refund (both federal and state together) is not what you received, you might have only received just your federal refund or just your state refund. These refunds will arrive in two separate transactions because they are handled by two separate agencies (the IRS and your state tax agency). 

If this amount was for only one refund (federal or state) and the amount was less than what was submitted on your return (after subtracting TurboTax fees), this means that they could have made a change to your return that has impacted your refund, or they could need more information to finish processing part of your refund. Another possibility is that it was partly taken in order to offset a prior debt owed.

In all of the situations above, you will receive a letter explaining exactly what happened, what additional information they might need (if any) and contact information. You could also call the IRS or your state tax agency before receiving this letter, they might be able to access your account and explain what happened.