Not sure which version of Sage you are using, but I have experienced similar issues with Sage 9. Some times the issue would be a wrong configuration in the resources/assets/config.json. Make sure that you have correctly set publicPath, devUrl and proxyUrl. If this is all correct, then try hard-reloading your browser. In Chrome, with Dev Tools open, click and hold the reload button and select “Empty Cache and Hard Reload”.