They may be fading as they age, young plecos always seem brighter to me than older ones. Or the lineage you have is not as bright as others. I've seen some super reds that looked more peach to me. Also, have you checked their lights? You may want to have them under a spectrum that maximizes their red color visually. As a last resort (IMO) you can look into food that contains additives. If you feed repashy, you can add their supplement (red rum) to the mix. Look for things that contain marigold or astaxanthin.