One room's convectors not heating?

All of our convectors are working beautifully, except for two in the same room. They are on a single loop in the basement. The output pipe is hot to the touch when boiler is on, the return is cool. One convector valve (coming off the hot pipe) spilled a bit of cool (old?) water when we opened it. The other convector did nothing (neither hissed, either). We're not sure what could be prohibiting the hot water from making it from the hot pipe into the convector.