WiFi connects to networks but not able to transfer data. What's wrong?

The Wifi on my phone is able to connect with available networks, but is not transferring data on it's own as it used to. E.g. I have to switch to Cellular Data every morning to see new emails that came overnight, new messages I received on my apps. Sometimes it connects and transfers data normally when I ask it to, e.g. when I open my YouTube app and play vidoes. But it doesn't do the same on other apps, e.g. won't download new messages even if I refresh. I'm not sure if it's a hardware issue or software issue. Suggestions? Thanks