When very distinct and sometimes heavy lines in the cross machine direction are observed in the fabric, they are often referred to as corn rows.  This is a phenomenon that almost all needlepunchers will experience at some time or another.  It is actually a harmonic for the particular needle pattern at a certain advance per stroke.  Every needle board pattern will have harmonics at certain advances per stroke.  They can range from very noticeable to hardly noticeable or unnoticeable.  At a very noticeable or apparent harmonic, the needles are hitting in or near the same holes repeatedly, which causes a noticeable pattern in the fabric.  The way to eliminate them is to change the advance per stroke until the lines or pattern goes away.  Harmonics can cause lines in the cross machine direction (corn rows) or in the machine direction.  Sometimes patterns such as diamond shapes or diagonals can be observed.  Again the cure is to change the advance per stroke.  In some cases, it is difficult to change the advance to obtain an acceptable surface appearance because of speed or throughput limitations, or the effect on other fabric properties, i.e. thickness, tensile properties, etc.  In cases like this, sometimes the needle efficiency can be changed so that the strokes per minute can be varied more to achieve an acceptable advance.  While more of a surface quality issue, corn rows can sometimes have negative results on physical properties.  This is especially true with fabrics made with brittle fibers such as glass or ceramic.