From Rex Parry...

If they are rising gently but are ignoring your dry….
Try dead drifting a lightly weighted size 18 pheasant tail nymph over the fish.  The take may be obvious as your leader is drawn under, but is much more likely to be really subtle – an inch of leader stabbing down, or moving slightly to a side, or just “hanging” slightly.  If you think there is anything odd, lift your rod.  Over time you will be striking without knowing why. This tactic works best on a calm section of water where you can see your leader on the water (rubbing the two thirds of your leader nearest your rod with Mucilin helps make the leader more obvious, as does having the sun in front of you).

If you have any top tips, please let us know!