Another thing to consider in your dicing problems is not only knife sharpness, but also blade thickness. I have a number of good quality knives that can dice onions smoothly. But when I use my sharpened Henckels chef knife (a relatively thick/hefty blade), slicing horizontally through the onion is not as smooth as when I use my MAC Pro chefs knife, or my other Japanese blades (e.g. Tojiro chefs knife.) When slicing horizontally, you are basically inserting a wedge in between each slice you make. The thinner the "wedge", the easier it will glide into the onion. I know...this is not scientifically accurate, but it gives you a generic-visual to refer to. And as one of the other posters mentioned, be sure to draw your blade through the onion and not simply push the blade into it or saw-cut into it.
One last thing that may open up a debate for better or worse: technically speaking, the horizontal cuts through the onion are not needed. As you cook the onion, the diced onions will not often hold their shape and will instead separate into single-layered squares. You can get the same results by simply cutting the onions with vertical cuts (stem-to-root) followed by perpendicular cuts (cross grain). Personally, I still do the horizontal cuts out of habit, as well as when I do fresh items (e.g. salsa) where the dice will hold up and maintain its shape. But if you are having difficulty with your current knife, you could always skip the horizontal cuts. Some will scream "HERETIC" at my suggestion, but try it...especially when COOKING onions...each diced cube will separate into individual layers and will look the same as onions cut without the horizontal slices. Good luck!