This is the problem i had with my three boys they were all brother and were fine for the first 8 months and then i noticed that one of them wasnt coming out much. I stripped down there tank as we use to use peat so they could burry if they wanted. when i got to the bottom one of the other gerbils i think had cournered another and it was to scared to come out. It had lost a little weight so i separated them as some times we could here squeking at night. He then starting eating better and did very well without his brothers.The other brothers were fine. I think alot was to do with the pecking order.