Welcome to ThingSpeak! What responses are you getting from the server on the serial monitor?
Also, I would make sure you can update your channel manually, using the command format for update a channel field.
https://api.thingspeak.com/update?api_key=XXXXXXXXXXXXXXXX&field1=1230
You can enter the format above in your browser address bar, use your channel write API key on place of XXXX...(not the user API key or the read API Key).