Why does my cooked onigiri rice get hard?

I've been working on my rice cooking technique; we don't have a rice cooker (and I don't want one, we have too many appliances already.) We have a good quality electric cooktop and I've been able to get improve my onigigi rice cooking to turn out a really good result. We also cook basmati rice and brown rice on the same cooktop with good results. But when I try to store the onigiri rice in the refridgerator after about 2 days the rice turns hard and it won't hold together when I try to make onigiri. The brand of rice I've been using is Rice Select and the type is "American Koshihikari". Its grown in the US. Is this a storage problem or a cooking problem? Phyllis