G64 needs only to be applied once and will remain in effect till changed. If it is in effect you should see it listed on your screen with the other active codes. Check to make sure it is there. You may also want to look at your acceleration. Acceleration, speed and line length all have an effect. Are you writing the code or using a program to create it? If you are starting with a CAD program and it contains a line composed of many small segments try combining the lines to one vector before converting.
but for many moves, especially short ones, the specified feed rate is never reached at all. Having short moves in your G Code can cause your machine to slow down and speed up for the longer moves if the "naive cam detector" is not employed with G64 Pn.