I think you're dissatisfied because you have such neat subject material and it's out of balance. The flower and everything behind it generates the most interest, and there isn't enough room in front of the flower to counter-balance that visual weight. If you've cropped anything from the left side, you might reinstate some of it , and tighten up the other three borders. If there isn't any more image to the front of the flower, you could Flop the image, tighten up the crops on the other three sides, and the viewing flow will feel more natural and forgiving in the new direction. You really have to see it, but it has to do with how we see and scan things in the Western hemisphere. Left to Right.
I used to grow cacti...'miniature ones' and this was identical to one I had...this spines are bloody sharp, so I hope you didn't get pricked?... Good contrasting work on this!..
Jen - Ignore any 'please do not touch' signs and the person gets what is deserved! lol The fleshy cactus and thorns takes on new beauty when the flower blooms. In this image, it is nearly centered, with the eye expecting more space to the left; perhaps that is what is the struggle with this image for you. Thad