On the R540 and road shoes, you shouldn't have any interference as mentioned by bradtx above. I also have those pedals and am right foot dominant, but I find it a lot easier to unclip with my left foot than right, but mostly because I've always done the left first and often leave the right clipped in at stops.
You may want to check to see if your cleats are angled about the same on both shoes and whether or not one foot points inwards or outwards more than the other. Other than that, it may just be a matter of practice. On one side you may be pulling more up or down instead of directly sideways to release. The mechanism releases with sideways motion, so if you're pulling a little more upwards or downwards as well, you are losing some of the sideways force that is needed to unclip.