I have a National Geographic map with a fair amount of detail; I know the little rivulet you're talking about, but that isn't the stream with Hen Wallow Falls - that falls is a fair hike away from that point, to the west. However, the trail to there starts in that area.
I would like to explore some of the feeder streams to Cosby in the Spring, when the water will definitely be higher. There are a number of them - Little Rock Creek, Inadu Creek, Toms Creek, all flowing into Cosby. I started down the Lower Mount Cammerer trail to check out some of them in June, but we had already fished, we were tired from the drive up, and we turned around before finding any water...but it's there, just a little further down the trail.
You're lucky to be so close to the stream...just keep fishing, and you'll get a bunch more. I do think the fish are a bit quicker about spitting out flies in the summer; I missed a ton of strikes on our last trip, and while some of them were my fault, there were more than a few where I know I reacted as fast as I could...very frustrating.