If the campaign is older than late 2016 it is possible that the custom BLP files it uses are no longer supported by WC3. A recent patch broke many badly made custom BLP files by changing the error handling of BLP files as part of a Mac OS crash fix. The old behaviour in Windows was to load all JPEG content mipmap levels, with invalid ones being replaced with default value 0,0,0,0 (transparent black) pixel data. The old behaviour for Mac was to try and load all JPEG content mipmap levels, with invalid ones cause an application crash. The new behaviour is to try and load all JPEG content mipmap levels, with any invalid one causing the texture to fail to load, and all models that use it to also fail to load (green checked cube). Obviously if the JPEG content BLP file is specified as having mipmaps and has a full set of valid mipmaps it will load fine, but many of the third party converters either do not do this automatically or failed to do this entirely. Newer campaigns will not suffer from this as the authors will notice and fix any such assets.
If the assets are valid but still do not load for you then make sure WC3 visual settings are set to maximum (some assets will not display at lower visual settings) and try restarting your computer. Note that shutdown (hybrid sleep) on modern OS does not count as a restart since only restarting will force the drivers to reload. It seems that graphic drivers from Intel and NVidia (no confirmed cases from AMD) can get into a bugged state and fail to render some WC3 assets after a very long up time. Reloading the graphic driver fixes this probably and is done by restarting the computer.