My federal refund has not yet come in

TurboTax does not get any information from the IRS after you file your tax return, so no one at TT knows why your refund was less.

You have not mentioned any specific amounts.  Sometimes we can make an educated guess as to the reason for your reduced refund if you mention specific amounts--for example if your refund was short by $1400---it is likely to be a mix up regarding the 3rd EIP payment.   If it is short by $1800 or  $1500 or  a multiple of that amount  ---   maybe a mixup regarding the advance child tax credit payments.   Or---the fees you were supposed to pay for using TurboTax + the "refund processing fee" charged by the third party bank….

Did you choose to have TT fees deducted from your Federal refund?  (Remember that if you did that, you also agreed to an extra refund processing fee of $39.99 or $44.99 in CA)

How can I see my TurboTax  fees?   

https://ttlc.intuit.com/questions/2565973-how-do-i-review-my-fees-in-turbotax-online

What is Refund Processing Service?

The IRS will send you a letter that explains why your refund was reduced.   Meanwhile it might help you to check your online IRS account.

https://www.irs.gov/payments/your-online-account

https://blog.turbotax.intuit.com/tax-refunds/why-is-my-tax-refund-not-what-i-expected-49240/