I am not sure about this,but I believe wix is a one page web application . So the browser progress bar will only appear when you load a wix website first time. Afterwards, the transition from wix to wix happens inside this web application and the browser might not show the progress bar. But if you reload the page, the web app is loaded again by browser from scratch and then it will show the progress bar. I hope this info helps.
To solve your problem, you could make a site wide pre-loader that shows when a new page is being loaded and hides when it is ready. You can find lot of examples here .
If you need my support to implement it, comment below or DM me.