This is kinda funny I came across this thread since my own Spider ball is having trouble putting on weight. He is 373g right now and just turned 3 years old on the 26th of August. Given his age and weight (yes I know he's underweight) he has a very bad problem eating sometimes. The first year I had him he would only eat once a month, which greatly hurt his weight gain/growth. I was thinking of trying to get him to eat 2 rats a week for a month to see how he responds, or possibly a bigger size meal like you used.