Browser loading finished before getting all the data from ajax calls?

I have an issue with data loading. I'm creating a dashboard with many Ajax calls. When I refresh the web page sometimes it works fine. But sometimes it rendered without some ajax calls ( means some data are missing). But I, again and again, reload the page it works fine.

Is there any method to fix this problem from JavaScript side or else. I want to do is showing loading screen until all data loaded into the browser.