My Wi-Fi QR scans but the phone will not join the network. Where is the problem?

Check what is inside the code first

Do not guess. Open the QR code generator, enter the same details, and look at the 'What is stored inside the code' box. That is exactly what the phone reads. Put it next to your router's settings page and compare three things: the name after S:, the password after P:, and the security type after T:.

The usual causes, in order

1. Network name: Cafe-Guest is not cafe-guest. The name is matched letter by letter including case, and one trailing space is enough to fail. Copy the name from the router settings instead of typing it from memory.
2. Security type: choosing 'No password' for a protected network, or WEP for a WPA2 network, makes the phone try the wrong way.
3. Password: a capital O instead of zero, or a lowercase l instead of the digit 1, are among the most common slips. And if you changed the password after printing, the old code still holds the old one.
4. Hidden network: if the network does not broadcast its name and 'Hidden network' was not ticked, the phone may not find it.
5. The router itself: some routers limit the number of connected devices or only allow listed devices, which has nothing to do with the code.

Special characters and Arabic names

The Wi-Fi format uses semicolons and colons to separate its parts, so a name or password containing one of them would break it. The tool handles this for you: it puts a backslash before a semicolon, comma, colon, quotation mark or backslash. Here is a network named مقهى الورد (Al Ward Cafe) with the password x;y:

WIFI:T:WPA;S:مقهى الورد;P:x\;y;;

The Arabic name stays as it is, the semicolon in the password is now preceded by a backslash, and the total is 41 bytes. If you typed the backslash yourself thinking it was needed, it gets added twice and spoils the password. Type the password exactly as it is and leave the escaping to the tool.