To address this issue, please make the following changes.
1. Admincp > Tools & Settings > User Agent Management > Manage User Agent Groups
Take the Google Mobile user agent and left click and hold on the far left area, and drag it above the google user agent.
2. Now edit the Google Mobile user agent, and set the regex to the following
3. Now go to your user agent mapping settings on the mobile skin. Admincp > Look & Feel > Manage Skin Sets & Templates.
4. Select make sure to select the following user agents to map to the mobile. Only the Bold Items should be needed, however for testing I used the following, so you can too if you see any issues.
Windows Phone 8
Save, and then test here - https://www.google.com/webmasters/tools/mobile-friendly
If it's not picking up the mobile skin properly, a few issues I ran into were server caches, re-caching the skins in testing worked to flush this. So if it's not working after the above changes, give that a try.