Depending on the actual errors, there could be many reasons for this:
- files aren't named with the proper case (usually uppercase first letter, lowercase for the rest), especially when developing in Windows, because Windows filesystems are typically case-insensitive, but the server is likely running a case-sensitive filesystem (since most filesystems for most other operating systems are case-sensitive, and most servers run Linux).
- .htaccess is ignored in some server configurations, or the settings need to be changed based on server software/version/configuration.
- you may need to adjust some configuration values in the /application/config/ files based on your server environment.