Hi, I can't answer for Toppik per se, as I use Nanogen's product (similar). Nanogen and I'm assuming toppik, is only held to your hair by static basically, so if you run your hands through your hair repeactedly, it will come off. I know Nanogen sell a locking spray which largely solves the problem, don't know whether Toppik do as well, and if they do if it's the same, but the Nanogen one works well. Couvre is fine an doesn't just come off unless your hands are wet (be careful after washing your hands though!) Although I've heard some people do say it came off when they used it, personally it didn't, I only switched to the fibre products as they look better.
Hair gel clumps your hair together making it look thinner, could be something to consider? I knwo Nanogen, so I'm assuming Toppik, say preferably don't use it, but if you do use it and let it dry first, then apply the fibres.