This late in the fishing season, most of the bigger bass will have moved deeper. Fishing pressure will also affect their willingness to bite. If you can go mid-week when the Bass and other fish have had time to settle down from the weekend fishing and boating, you'll proabably have much better luck. Shade is a good attractant to bass and other fish, so if you can scoot your flys under low hanging trees, under docks, and in secluded nooks in the bank you should do better. Best thing would be to get to the water while the sun is just glowing on the horizion. This will be the coolest part of the day and the most likely to get a top water bite.
Finally, most fish, including bass get lethargic and unwilling to move much for a tiny meal, especially in the middle of summer or winter. I'm not a fly fisherman, but if you could add more volume to your baits, such as feathers that make it look bigger but still let it float, that may present a meal that a big bass will go for.