"I did not live with them the full year however, so I'm not a qualifying relative."
That statement is wrong. The rule is: Closely Related OR live with the taxpayer ALL year. You, almost certainly, can be their qualifying relative dependent.
The most likely reason for the delay (or non payment) of the stimulus is that you parents did claim you. Verify that that didn't happen. Otherwise, yes the IRS has red flagged your payment because you appear to qualify as a dependent and you were a dependent in 2018.
But, in regards to the stimulus payment, when you file your 2020 income tax return, in 2021, you will be asked for information about the amount of the stimulus payment you received, and if it is less than you are entitled to, you will be given a credit on your 2020 tax return.
"In essence, the stimulus check acts as an advance of your 2020 income tax refund. This means when you prepare your 2020 income tax return, there will be a line to include the section 6428 credit. The credit on your 2020 return is subtracted by any amount received as a stimulus check in 2020. If the amount you received as a stimulus check is less than the credit you are due, the difference will be included as part of your 2020 refund. If you have been overpaid by receiving the stimulus check, however, you will not be required to return any excess amount".
