I'm no expert, but there are a few things that come to mind that might be causing problems here. The first is that I have read (but haven't experienced myself) that some rennet tablets are not particularly good for cheese making. Don't know if that's entirely accurate, as I said I've never tried anything with tablets. Another thing is that, at least for liquid rennet and I believe this holds for tablet/powdered, as well, you want to use cool water (the recipe doesn't mention this) and you don't want it sitting around too long before it gets used. If you used warm water and then let it sit for 30 minutes as you continue to heat up the milk (as the recipe seems to suggest you do), the activity of the rennet will be reduced.
Is your milk really directly from the cow? If it's raw milk that isn't really fresh, it might have too much acid in it. I had that happen once with some "fresh" goat milk that was actually a few days old. After adding the rennet and stirring a bit, I could see little flecks showing up in the milk. After letting it sit, there was definitely a curd, but it was rough and tough, not smooth. This was because I had too much acid. If this is the problem, try reducing the amount of citric acid you add.
I also don't know why the recipe calls to dissolve some of the acid and sprinkle some onto the milk directly--I'm just not sure why you'd do that.
Those are my thoughts. Someone else might have better suggestions.
Finally, you might want to check out some of the recipes on the forum and use them, instead.