As an example, an excerpt of the google API is as follows:
<condition data="Scattered Thunderstorms"/>
If you are keying off 'condition' and not the 'icon', the 'condition' may not be a supported value. If you key off the 'icon' it should be able to resolve the icon easier, even if you are remapping to your own icons.
BTW, why don't you use their icons? They look OK to me.
The city I'm reporting is Minneapolis, MN. Wednesday is the day that doesn't have an icon(see code block)