We also have another mobile extension which has already have this detector plugin and a built in mobile template: Flatlia. Check it out here:
The detector script is based on mobileESP:
How To Install
1/ Download the plugin by clicking here: https://www.blackreddesigns.com/store/products/1-flatlia-mobile-extensions/3-joomla-mobile-detector-plugin
2/ Install it as a Joomla! 2.5/3.3 extension
3/ From Plugin Manger -> find the plugin (Mobile Detector) -> Enable it + Make sure that the option "Enable Template Switcher" set to YES - Select the template to apply when switching
4/ Save it. Then use a mobile device and browse to your Joomla! website to check and see that the switcher is just work!
I am using 2.5.9 and installed the plugin at the last position as detailed in the documentation.
We have just released a new version with may fixed your issue. Try it now :)