Retirement tax questions

Thank you for your help! Since I'm here, and you all know a whole lot more than I do about these things, could you possibly give me some insight on the following? The IRS sent me a letter saying that I'd get only $600 for this latest stimulus check. I make well under 30K and have for the last 9 years. My AGI is only 27,000 something! Practically living on the edge of existence. I collect a pension and SS. Why am I not getting the full amount I am entitled to? I fill out a simple tax form, have no investments, have never deferred anything....easy peasy. What am I supposed to do now? Just wait and see if the IRS notices the error and corrects it?? Thank you for any understanding and suggestions you can give me. -Chrissy