I work with credit. Just because you get a letter saying you are pre approved, doesn't make it so. Think about it. Banks and other businesses blanket enormous amount of people and businesses, giving them the opportunity to get a loan or a credit card etc. They have to look at the individual credit history.
Let me ask you this: how is your income to credit ratio? For example: you make $100,000/yr but you have $75,000 in credit. (you might not even be using all that credit, but you have it in credit cards or loans) That right there is too high for alot of companies to allow you a loan.
The best thing for you to do are two things:
1) check your credit and find out what your score is.
2) call the lender up and ask why you can't get a loan through them. They must tell you why they won't.
3) if you have a bank you deal with or credit union--ask them for a loan or at least get information. You can't get too much information on that. I would go to at least 3-4 sources and see which loan is best suited for you.
working with credit has opened my eyes up!!