Text to speech

My iMac's operating system is Mojave and the default command for text to speech is command plus M. This works fine when I highlight a page on Wikipedia.but when I try it on an eBook, nothing happens except for a pause followed by a low dong sound which I think means "** off I'm not going to read this". Sometimes it works OK with Microsoft Word Docs, sometimes it doesn't. The same with pdf documents. Any idea how I can I can get a more reliable response? At the moment it's so unreliable it's useless