My guess is that your text is not directly under the 'content' of the other layer. Either that or your text is exactly the same color as the 'content' of the other layer.
Remember that with a clipping mask, the non-transparent content of the base layer (your text in this case) reveals the content of the layer above it. So, if there is nothing directly above the text, the top layer will 'seem' to disappear. That would also happen if your text was the same color as what's directly above it. And it doesn't have to be text by the way :-)
Try it again, but this time make sure your text is directly below some content on that upper layer and that your text is not the same color as what's directly above it.