Just to clarify the terms used in game - when you use your right mouse button to stop an incoming hit it is a parry, it stops all damage, and it gives you an instant counter that does high damage. If you have to hold that position for more than about 2 seconds, it becomes a block instead. You take a little bit of "chip" damage and don't get that counter.
First of all, go to your settings and play around with the various combat settings. Some people like keys, some click and drag, some like drag and click. Try them all and see what works for you.
Now adjust your mouse sensitivity. The default settings are way too high, so reduce them. I find 10% general and 5% for combat works, but you can go as low as 1%, whatever you find best.
Finally, the question you actually asked about parrying to the right. Even after all the time I've played I still find that if I'm going to mess up a parry, it will be the one to the right. Left, high and low are always fine, but right sometimes screws up. I thought it was just me and the way I held or moved the mouse, but I've now spoken to lots of people who have the same issue, only on the right parry. I've also taken extra care to watch exactly how I move the mouse. It makes no difference - it seems to be a genuine bug with the game that affects just that one movement.
Adjusting your settings, and doing lots and lots of practice will mean that things work most of the time, but that annoying problem with right parry seems to be something that just happens occasionally.
