Did you copy and paste into the app? The word count updates based on typing, so if work is pasted in, it won't update until you start typing. Or perhaps it's not correctly recognizing your keyboard. Could you tell me a little more about your set up?
I just released an update, could you please install it and let me know if it fixes the issue? I recommend using the itchio app to run and install StimuWrite because it keeps it up to date and solves some of permission issues that come with unsigned apps.
Something I've noticed is that a lot of the program's features--like the em-dashes and the wordcount tracker--stop working if you're not writing at the end of the document. If you write two pages, then realize you had more to add at the end of page one, the wordcount won't update until you type something down there. IDK if that's your issue!