Welcome to the forum. Try deleting the saved networks from the phone.
Random connection problems are regularly reported here and there's no standard fix. Clearing stored networks is one that has worked for others (making sure you can easily get the wifi key for any you want to use again). Reinstalling the app can also do it, and there may be other suggestions.
For what it's worth, my glitch some time ago was that one device simply refused to connect while the app was installed, and I had to connect manually. It was cured with the next app update.
I always find the BT WiFi Android App completely illogical.
Normally I will connect to the strongest WiFi signal that I have access. If it is BT WiFi then I would like the App to log on for me which it occasional does but usually it wants me to start up my GPS so it can look up the nearest hotspot. This of course is not of any use of I am inside a building as I tend to be when using the tablet!
I cannot understand why it cannot first search for the best signal and connect to it.