The most common cause is a high viscosity coating that will not wet-out. Add a surfactant to the coating or increase the time between coating and drying if possible. If not, consider an alternate rod type, such as gapped or double wound rod.
The rod is likely clogged with dried coating or some other contaminant. Make sure the coating is pumped through a fine enough filter. Rotating the rod also helps evacuate any particulates. The rod needs to be cleaned anytime coating has been allowed to dry on it.
The life of a coating rod depends on the abrasiveness of the coating and substrate, the hardness of the rod and how well it's maintained. Either the tops of the wires will wear down from abrasion or the grooves will clog up with coating. If the rods wear out consider chrome plating. If the rods clog up consider an alternate rod type. Check the pressure between the rod and the backer roll (if applicable).
Check the web tension across the rod (if applicable). Ensure that the rod is aligned with the backer roll (if applicable). Check the straightness of the coating rod and replace if bent.