Once all the required documentation has been submitted it will be reviewed by the underwriter. The underwriter has the authority to approve or reject you for the loan. Lenders have certain guidelines for who they will and will not lend money to. Sometimes even individuals who have been pre-approved for a mortgage may be rejected once their financial documents have been looked at closer. Their income may be too low, ratio of debt to income may be too high, the person may already have too much debt, the credit score could be too low, or the person could otherwise be a bad credit risk. Lenders want to make sure you can make the monthly mortgage payments, have a good credit history and have collateral for the loan.
If not rejected, many borrowers are given conditional approval at this point. This means that they are tentatively approved pending the satisfaction of certain conditions. If the issues are minor and can be resolved in a short period of time then the mortgage can move along in the approval process. Some lucky borrowers go through this stage without any issues.