If the IRS reduced your refund, they will send you a letter explaining why. You probably won't get the letter until a few weeks after you get the refund. You can't attempt to "fix" it until you know why they reduced it. There might not be anything to fix.

How much is missing?  A common adjustment is if you claimed the Recovery rebate credit on 1040 line 30 when you already got the Stimulus payment or got the second one after you filed or reported the wrong amounts or entered them in the wrong box.  Do you have an amount on line 30?