No data returned, at same time during the day

I am a student, I've chosen to use WU, instead of the the other API's which were suggested to us, because of the amount and different type of data you return. However, I usually code at night/late afternoon (my time NZ) and I find I have no data to work with from mid-afternoon. I'm using Xamarin Android with c# in Visual Studio. I know my chosen API (JSON) is working because I have no problem receiving the data during the morning and early afternoon. As far as I can see my daily hits are well below 500 so not sure what is going on, if I'm doing something wrong, then why would it work sometimes? or if there is a problem at WU. 

Please help, my assessment is due end next week and with Easter celebrations, I have very little time to get this right and soooooo much work to do........

http://api.wunderground.com/api/{my key is inserted here}/geolookup/conditions/forecast10day/q/NZ/" + txtCityName.Text + ".json";

The txtCityName.Text would be Hamilton.