I know it doesn't solve the real problem of not being able to get rid of them with a gravel vac, but those osmocote balls will empty of liquid over time if in the air, and then will take some real time to fill back up and sink again, so if you're trying to clean these guys out, the easiest thing to do would likely be to scoop out the densest collection of substrate/osmocote balls, let them dry on a sheet, then re-submerge after a handful of hours/days. (no idea how long) the osmocote balls should simply float to the surface at that point.
Not a great solution for an established tank, but not a bad means of sorting them out of substrates like sand once you're re-starting a tank. I found this also worked for helping me seperate organics from my gravel. In general organics, once dried out, won't resaturate quickly, so if you leave your substrate out in the sun for a day and then dump it in a bucket of water, and gently stir, all the nasty floats up and the substrate stays on the bottom. Just thought I'd throw out my two cents. It's not flawless, but I hope it might be helpful to some.