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What are their signs of "failure to thrive?" Are you weighing them regularly?
I'm not sure calf manna is a good feed for them. Its principal ingredient is soy, which is not recommended for guinea pigs. It also has a minimum of .7% calcium, which is high for guinea pigs, and may result in urinary tract stones. Particularly in conjunction with oats, where some varieties are also high in calcium, they may be really getting too much.
A good diet for guinea pigs consists of unlimited long strand grass hay (timothy, blue, meadow, orchard, brome), good quality plain pellets, and a cup or so of fresh vegetables, of which red or green leaf lettuce and bell peppers should be staples. They also can eat tomato, young green beans, small chunks of carrot, summer squash, occasional other greens. Anything high in sugar should only be an occasional treat, as guinea pig intestines aren't really designed to process sugar. You can see
https://www.guineapigcages.com/foru...vy-Nutrition-Charts-amp-Poisonous-Plants-List for what guinea pigs can eat and how often.