Hiya new to this... about 2 months ago I bought 4 little piggies for my children they were both fine for weeks then out of the blue 1 of them was very quiet didn't really move much and was lying around most of the day so I put him in a cage on his own to make sure that he was eating plenty & drinking. The next day the greedy one of the bunch in the other cage was dead! I have no idea why he died but non of them seem to be putting on weight! They are all boys and all from the same litter and kept inside in large cage.they are 3 months old. My mum has a female from same litter and hers is double the size help...

What are you feeding them? Guinea pigs and humans are the only animals that can't make their own vitamin C, so if you aren't feeding something specifically for piggies, they may not be getting the right nutrition. Other than that, it may be worth going to a vet on this.

Here's a link I found that might help. I didn't read the entire thing, so I cannot say with 100% certainty that it's reliable, but I'm sure you can decide what makes sense and what doesn't.

Animal Health Care - Keep Your Guinea Pig in Good Health for Life

Remember that Guinea pigs are considered an exotic pet, not all vets will know about their care and their needs.

Good luck.