Not doing anything wrong - wildlife is wildlife and doesn't always follow a script. A big flock int he distance was possibly wheeling about because someone had walked too close (happened on Sunday), before returning a little later. Other thing with wildlife, often takes a bit of patience...
Aberlady is soooooo much more than just that bit by the road as well. There is a stunning, huge, beach at the end of the walk through where you can see Sanderling and Ringed Plovers and Gannets and Cormorants and Seals (and rusting single person subs once used for bombing practice); walking there you go through grassland and moory type stuff and sort-of-machair and buckthorn bushes that are full of Fieldfare in the winter; you can see Deer and Peregrines and Buzzards, Kestrels, Damselflies, Dragonflies, Stonechats and more more more.
The 'dull lagoon' attracts Little Egrets and Curlew, all sorts of types of ducks and geese.
Occasionaly a White-tailed Eagle will fly over (never seen that one myself).
Aberlady is truly a wee gem of a place.