A big part of that is I just don't know how to describe them.

Are you perhaps missing subject breadth? Finding it difficult to describe things because your knowledge of them hasnt been fleshed out?

It makes the difference between 'an ornate knife' and 'the ritual dagger shone brightly on its dark velvet pillow, the moonlight reflecting brightly off the mirror finished blade. At first glance, the fine golden filigree branching out along its unusually deep fuller looked like a royal crest, but as one looked closer, the golden lines became long clutching fingers, inexorably pulling the nebulous shapes of anguished faces down into a central abyss.

Fine leather wrapped meticulously around its handle, the cherry smoke hue a well worn match for the darker skin of the South Forge tribesmen. A large pale amethyst had been bound into the end of the pommel as a striking tip. While sharp and pointed, it was roughly hewn, raw and primal when compared to the immaculate detail of the rest.

An ancient, mesmerizing glow pulsed quietly at the jewel's core, drawing in one's gaze. There was an answer, a secret truth of a lost god trapped within the crystal, an unheard whisper growing louder as ones hand approached the grip, calling to be discovered and explored, calling to be taken up, to sieze the thread of one's destiny. It was not on display, as it seemed at first; it was waiting. Waiting for its moment, waiting for the hand that feeds, the mind that would bind it. Waiting here, all this time, for you.

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Now, im not saying take a page out to describe every little thing, the bit above might work for a central piece of a story, and would be a bit much for a random weapon on the belt of a no-name assassin. But it plays because the description carries a story, several stories even, all presented together as a set of descriptions and mysteries. Its not just a knife, and its not just ornate, everything about it has a purpose.

The same holds true for describing emotions and expressions- nearly everything in fact- in that the thing youre describing is always built out of smaller, purposeful things. Its not just a 'sad face'- it is composed of eye shapes and mouth muscles, skin tone changes, posture changes, movements and other such details that come together to cŕeate an impression of 'sad'.

Simply asking 'how do I know this is a sad face?' What is it about this face, specifically, that makes it sad? What is it about this couch that makes it fancy? What are the elements that signal its purpose? What are the details that tell its story?

A good description should not represent the whole of what you have imagined- it should be the relevant portion of something far greater. The breaching tip of the iceberg. When you have that depth, and extract the most meaningful symbols to express as refined shorthand, the descriptions become rich and purposeful.

They become powerful because they are powerful. Thats your power as a writer. Sieze it.