This is good advice.^
Channel catties can indeed be aggressive, but getting them to eat a fly can be very hot or cold. Much of the time, they aren't in the mood. However, in my experience, when you see a cattie in shallow water with current (as Krayfish describes above), look carefully - there are likely multiple others in the vicinity. When catties are laid up like this in riffles, they're often hot to trot. Although it's hard to see 'em in low light or cloudy weather, they are more likely to eat under such conditions. For this reason, dead drifting or swinging streamers in current on summer evenings can hook you up with Mr Whiskers. If you stick a cattie, keep working that spot as there are almost certainly others nearby willing to eat.