Nokia Universal Portable USB Charger DC-18 - I charged my DC-18 from a wall outlet and at some point I plugged the micro-USB cable to my phone to charge DC-18 and my phone at the same. Why didn’t my phone start charging?

Your phone starts charging immediately when DC-18 has enough power (typically above 75%). If DC-18 doesn’t have enough power, DC-18 charges first, until it has enough power to charge the phone.