Here are a few things you can check: 

    Verify that the aura:if condition is correctly evaluating to true when the combobox should be displayed. Double-check the condition and make sure it is evaluating as expected. You can try temporarily removing the aura:if condition and see if the combobox appears correctly to confirm whether the issue is related to the condition or the combobox itself. 

    Ensure that the necessary dependencies and attributes for the combobox are correctly set up. Make sure you have imported the required namespaces and included the relevant Lightning component dependencies in your Aura component's markup. 

    Check for any JavaScript errors or console logs that may be occurring when the combobox is within the aura:if condition. Use your browser's developer tools to inspect the console and see if any error messages or warnings are being logged. This can help identify any potential issues with the combobox or its surrounding code. 

    Review the code within the aura:if block and ensure that there are no conflicting attributes or components that may be interfering with the combobox's functionality. Check for any potential naming conflicts or issues with data binding. 

 If none of these steps help resolve the issue, it would be helpful to provide more specific details about the problem, including the relevant code snippets, any error messages or warnings you are encountering, and any additional context that might be relevant.