I have a similar problem in that I connect to the WiFi OK but the app doesn't connect to the drone. I have discovered that this often clears as soon as the drone gets airborne. Plus, as Holtneil says, the WiFi has a tendency to reconnect to my home WiFi or stronger WiFi if one is nearby.
I'm assuming that, in my case, the 'phone or tablet needs a clear line of sight to work properly, with the drone away from the ground. These apps can be quite flaky, in my experience.
Have you tried flying your drone and seeing if the connection suddenly makes to the app?