Credit score 600, have 20% Downpymt,over 100k salary but DECLINED

[Cynthia Walker]

Very frustrated... we have fair-poor credit, excellent employment, and want to buy a house $399k, can put up to 20% down, buying a primary residence, but cannot get even a pre-approval letter.  What are we doing wrong? We are able to pay off necessary items to rapid rescore over 620 but still cannot get a pre-approval letter for our dream home that’s about to slip away today.  How do people who have the same credit, more debt and much less income buy houses???