OK, here's the answer, I believe. actxGetRunningServer() will not find a running instance of either Excel or PPT if you have installed 32-bit Office but are trying to execute from 64-bit MATLAb (presumably the other way around too). However, since multiple instances of PPT are not allowed, you can't just start a server up either without shutting the existing one down (of course, that doesn't apply if you started PPT in MATLAB). This restriction does not apply to Excel as multiple instances are allowed; therefore, even though 64-bit ML won't attach to 32-bit Excel, you can just start another one. actxserver() doesn't seem to care about 32-bit 64-bit mastch up?. At least all this seems to apply for R2015b.
I note that the doc for R2019a doesn't say any more about this than what I have for R2015b.
