There can be many reasons as to why they are not getting installed properly or completely, if they aren’t getting installed, then the problem might include but not be limited to:
1. You are using 1.0 components for 1.5, without enabling legacy mode. Goto plugins manager in Joomla! 1.5 administration and enable the legacy mode plugin to be enabled to use the Joomla! 1.0 extensions(modules, components, plugins, templates) on Joomla! 1.5
2. You are uploading in a wrong format (e.g .rar instead of .zip). Mostly servers don’t allow upload of rar file format and the unzip it. So use zip format for this problem and make sure you are using the correct format compressed file before uploading.
3. The installation file may have many problems. If you have coded the extension yourself, or have got it from other source too, there is possibility that there was some error in the installation xml file. Please have a look at the error carefully and see if there is some problem or error in the installation file for the extension.
4. Your server settings are wrong, e.g ftp login and password. Goto Joomla! server configuration, and enter the correct ftp login and password settings and test them to make sure they are working properly. Normally if you enter the ftp details and then select to enable the ftp layer, and then hit save to save changes to the configuration file, if it goes successful, then it means that the entered ftp details are correct, otherwise they might be in correct (except for the case when configuration.php file is write protected)
5. Directory permissions are not configured properly. Simply make sure that the directories that need to be writable are writable (chmod 777). Specially tmp, cache, components, modules, template, plugins, and the same folders in the administrator folder. Make sure to turn the directory write permissions to read only (chmod 755) once the installation in complete, so that there is no security hole left open for your Joomla! installation.