My only thought was that it dried out, but you say it's damp. Are you sure? Do you probe the soil with a wooden dowel or skewer or something? What is your growing medium? Zone? How long have you had the plant?

Hi, I had it for nearly 3 months. It's in acidic azalea compost. The garden centre recommended it. I put my finger down around around the root ball.
Now I noticed small brown spots on the normal looking leaves.:-(
Shame I really wanna save it.

This looks like a disease to me, possibly something growing in the soil that is interfering with the roots. All of our blueberries are planted in the ground, here in Madison, Wisconsin, where I live. However, I do pay attention to posts made by people who grow them in pots. One recommended soil mix is 50% peat moss and 50% pine bark fines(shredded pine bark). The pot should have several drainage holes, and it should be slightly elevated off the floor, so it can drain well. About once a week, the pot should be overwatered, so that considerable outflow happens at the base of the pot. This is done to "flush out" any slimy bacteria, considered undesirable, that might be colonizing the lower reaches of the pot. Some growers overwater every time they water, some people do this operation once a week. I hope this helps.

Eric is 100% right. Compost in pots is not recommended. Mine are in pots too, and the mix is pine bark mulch and peat.

Thx guys. I'm gonna get some bark today. I hope it's not all lost.:-0