After you file

If the IRS decreased your Federal refund, you should get a postal letter from them in about 3 weeks or so explaining why your Federal refund is less than what you filed.  The letter should also tell you steps you can take if you disagree with their decision, including a contact number for you to phone them if desired.  The following help article will tell you a bit more about possible causes, how to check for any offsets, etc.

https://ttlc.intuit.com/questions/1894477-why-did-my-federal-tax-refund-decrease-after-i-filed

The IRS has said:

"Examples that could decrease your refund include math errors, mistakes, delinquent federal taxes, state taxes, child support, student loans or other delinquent federal nontax obligations, and when the IRS withholds a portion of your refund, if further review is needed."

If you want to talk to the IRS before you receive the letter to see if you can learn anything, here's how:  800-829-1040  hours 7 AM - 7 PM local time Mon-Fri

• First choose your language. Press 1 for English.
• Then do not choose the first choice re: "Refund", or it will send you to an automated phone line.
• Instead, press 2 for "personal income tax".
• Then press 1 for "form, tax history, or payment".  
• Then press 3 "for all other questions."
• Then press 2 "for all other questions."  It should then transfer you to an agent, but there is likely to be a long wait.