Who can I talk to to make sure my refund was correct and to make sure I’m not missing any money that I’m owed?

My refund was estimated to be over $1,200 this year, but when I did my taxes, I only got under $350 back, and I feel like something is missing from my taxes. I've already go the refund but Who can I talk to to double-check if I’m not missing any money I'm owed and how do I request to redo my tax refund if possible?