How do you have them set up? What type of enclosure? substrate? hides? temps? Are they housed together or seperately? Is the enclosure near any drafts of cool air? Are they near the floor or higher? Do you handle them constantly? How about a run down of what you are doing to try and help eliminate what you may not be doing?
The most obvious question is when(time of day) are you offering the prey items? alive or frozen/thawed? Do you move them in any way to feed them? Are you patient when offering food(allowing the Viper to approach), or do you corner them with the feeder(holding the prey against the head and body); applying to frozen/thawed? If offered alive feeders, is there room enough for the Vipers to choose not to interact?
I have had certain wild caught Vipers take up to 3 months of being completely left alone before they began to readily accept prey(provided sufficient weight, heat, and clean water). Stress is the biggest factor with Vipers. If they do not feel secure, most will not settle in.