If there's a partial blockage in the drain line, that might cause it. There's one closed cell foam wax ring replacement product some people here have used and liked...it should be more robust in that regard.
But, the biggest candidate is that the toilet may need shims to keep it from rocking...the wax isn't springy, and once you set the toilet, any rocking will cause the wax to compress, and when it rocks back, leave a gap. It doesn't take much. Set the toilet in place without wax, and if it can move at all, figure out where to place shims to stop it, then add the wax, and set it on the shims. If the floor is pretty close and the toilet isn't badly warped, I find that an assortment of coins, penny, nickel, or dime, is all it usually takes to solidify things. You can make a trip to the store and buy some composite shims, and those may be needed if the gap is bigger than an astutely placed coin or two will resolve.