Device detection doesn't work
This problem could happen if you are using cache on your website. Most caches save down the website's code as it is, and they doesn't check on what device you are. The device detection is a data coming from before the page loads, but if the website is saved down, that code won't load, so your website will show the same desktop display everywhere. There are cache plugins/extensions (for example: JotCache), which are using device based caching, and you should use one of them, or choose only one, to either use device detection or cache.