What are the ammonia levels? You didn't say, and that could be a possibility. When was your last water change, and how much do you change each time? Your tank isn't overstocked, if you ask me, so that rules that out. Your pleco's temperment is peaceful, so I doubt it is a bully. However, plecos tend to be noctural (at least in general - I don't know specifics on individual species) so perhaps take a peak at your tank at night see if you observe any bully tendencies.
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