The expression 'ไม่ใช่เพียงแต่ (mâi châi phiang dtàe)' is used when you want to convey that something is not only the case, but also something else is true as well. This is similar to the English 'Not only...' construction. It is often used to indicate that in addition to one thing that may be obvious or expected, there is also another thing which might be surprising or additional in some way.