A little loft dampness

I have just put some items in the storage area in the loft and noticed that the black felt(?) stuff on the loft ceiling was damp as were some items under it. Not the whole loft was damp just a smallish part or at least that was what I noticed. I just want to know if this is a problem I should take action on? I had a couple of ventilators installed a couple of years ago, it is reasonably insulated.