Hey Joan,

Maybe try it this way, if your project has summary tasks you'll notice next to then they have plus and minus signs. If they have a plus sign you can click on it and it will expand out and show all the sub tasks underneath that summary task. See screenshots below for reference. Although expand all should just expand everything out in one click. On my end the feature is working so there might be something project specific that's causing expand all not to work correctly. 

First try my workaround and if that doesn't work it would be great if you could share the project file with me so I could take a look at it. 

Thanks and Regards, Sophia