Hi Phil,
Don't beat yourself up over not catching.
I don't think you've been doing anything wrong, from your post you have only spent 6 hours on the fishery, which isn't a long time really, but you've had a couple of takes even though the fish got off.
In a 2 hour session you'd have to fish incredibly hard, and make quite a few changes to try and work out what the fish are taking and what depth they are taking at? Bear in mind, each time you visit this could be different.
While you probably aren't filled with confidence at the moment, try doing a longer session (5 or 6 hrs) from first thing in the morning, which will give you time to relax, and also so that you have enough time to experiment and try a range of depths and methods.
If there is nothing moving on the top, start fishing in the surface layers (2 to 3 ft deep) and work you way down to the depths, alternatively, you could start deep and work your way up.
If you are fishing a cast of 3 flies, try 3 different flies to hedge your bets. A lure on the point, nymph on the middle dropper, and a black & silver (or balck & gold) buzzer on the top dropper, and then make regular changes of fly, etc, every 10 to 15 minutes, having fished through various depths.
Also, the speed of your retrieve can make a big difference. On certain days they may want it completely static, other days stripped back at speed, so try a range.
If another angler is catching, watch exactly what othey are doing, with regards speed of retrieve, and any time waited before starting a retrieve - as this will give an indication if they are fishing at depth, etc.
Next time you're struggling and you see someone consistantly catching, go over and tell them you're struggling, and politely ask them; what fly they are catching on, what depth, and what sort of retireve? Most anglers will help you.
Having said all the above, this is the hottest time of year, water temperature will be the warmest they have been all year, and that could put stocked rainbows off the feed, especially in a small fishery when oxygen levels are low.