First and foremost, not all plecos share the same diet requirements. So saying whether plecos can survive on just wood alone without determining the genus/species is vague. While there are some plecos species that are predominantly wood eaters (Panaque and Panaqolus), there are others that are carnivorous (Pseudacanthicus), herbivorous (Ancistrus) or omnivorous in nature. As for the one pleco that is not eating, it is possible that the dominant pleco is preventing the subdominant pleco from eating. Plecos are dominant/territorial fish and they can fight when kept in small tank. As for whether or not a pleco can survive on wood just alone, I say no. Like I stated earlier, not all plecos share the same diet requirements. And even if you have a wood eating species, offering vegetables from time-to-time would benefit their health instead of just relying on just wood alone, even though wood is the main part of their diet.