Black Pennel scorned

I usually find that a Black Pennel is a never-fail fly on most of the Assynt lochs, but in the evening of 28th May 2015 it failed on the Maiden Loch. I fished on the road side from where the loch narrows, round towards the Lochinver end. I maintained a Black Pennel on my cast throughout, but the three fish I landed, and returned, (from about 4 to 12 oz) all took a Clan Chief or a Blue Zulu. Clearly the Maiden Loch trout like fancy flies.