This is probably because the icon pack you installed does not have an icon for those apps. Icon packs only include app icons for certain apps and clearly the one you've installed does not cover many common apps. If you download another icon pack you may find that includes more icons for the apps you have installed.
Another option would be to contact the developer of the icon pack you have installed and request that they include an icon for the extra apps.
If the icon pack does include an icon for the app but for whatever reason it doesn't change, you can do this manually.
- Long press on the app icon
- In the menu that appears, click on 'Edit'
- Click on the icon in the popup
- You should then be able to see all available icons, from which you can select the one you wish to use for that app.
This will change the icon everywhere within the launcher (i.e. app drawer and home screens).