Hi and welcome
Guinea pigs can go off their pellets; you can try to mix in another brand and see whether that catches their interest. if not then I would start looking for medical reasons if it is connected with a weight loss, especially dental or pain issues.
Please switch from weighing weekly to weighing daily at the same time in the feeding cycle (i.e before their dinner or breakfast) with any guinea pig you are worried about. Please make a vet appointment if the weight loss exceeds 50g (2 oz) and see a vet promptly if the weight loss is 100g or more (3 oz).
Start offering syringe feed and water promptly to support your guinea pig until and during any medical treatment if there is a major loss of appetite/weight.
Complete Syringe Feeding Guide
The Importance Of Weighing - Ideal Weight / Overweight / Underweight
Recommended vets in
Guinea Lynx Records :: Topic - Veterinarians: Pennsylvania
We would like to ask you to please use grams on this forum, as we have members from all over the world. US metric pounds and UK imperial pounds are not the same. The difference especially in the guinea pig weight range can lead to confusion and wrong advice being given.