The method of applying thin-bed and medium-bed mortars has changed over the years. Today many installers use (or should be using) the straight line troweling method as described in the National Tile Contractors Association (NTCA) Trowel and Error video.
This method requires that the mortar be keyed (or forced) into the substrate with the flat side of the trowel and then notched in one direction, either left to right or up and down. The tile is then moved in a perpendicular motion to the mortar ridges. This back-and-forth motion lays the ridges into the valleys, normally yielding full mortar coverage on the back of the tile. This process has been in use for well over 20 years and has solved many of the mortar coverage issues previously found in the field.