dependents filing tax return to get tax withholding back

I have 2 full time students as my tax dependents for 2021.  When they are filing 2021 (intending to get their withholding tax back) in TurboTax with W-2 income less than $4300, only one of them can get tax refund for taxes being withheld in 2021 but not the other one.  Any idea why not both of them getting tax refund?