My curry tastes flat even though I followed the recipe — what is usually the missing thing?

The most common cause by a wide margin is acid, and it explains everything in your description including why salt half-worked.

Most home curries are rich, sweet-ish and savoury with nothing sour to push against. That combination reads as flat no matter how much spice is in it, because there is no contrast. Salt helps a bit because it sharpens perception generally, then hits its own ceiling — which is exactly the pattern you described.

The fix is a squeeze of lemon or lime at the end, or a spoon of tamarind, or yoghurt stirred in off the heat, or a splash of vinegar. Start with less than you think. The target is not "tastes sour", it is the point where everything else suddenly gets louder.

This is the single most reliable rescue in the whole category, and most recipes leave it out because the writer adds it by instinct without noticing.

For question 3 specifically — rescuing one that is already flat — in order of what to try:

1. Acid, as above. Small amounts, taste between each.
2. Salt, but only after the acid. The amount of salt that tastes right changes once there is acid in it, and adding salt first is how people end up salty and still flat.
3. A finishing aromatic off the heat: fresh coriander, a little garam masala, ginger julienned in at the end. Whole-spice aroma that has not been cooked away.
4. A tempering — a spoon of hot fat with cumin or mustard seeds or dried chilli poured over at the end. This adds the bloomed-spice layer the dish may never have had, and takes ninety seconds.

That last one is the closest thing to a universal rescue, because it is adding the step that was probably skipped, after the fact.

The second cause, and it is the one that produces the dusty result when you add more spice: the spices were never cooked properly.

Ground spices need to spend time in hot fat. That step — blooming — is where the aroma compounds are released and dissolve into the oil, which then carries them through the whole dish. Spices added to liquid, or added late, stay as suspended powder. They taste dusty because they literally are dust.

So when a recipe says fry the paste until the oil separates, that instruction is doing real work, and the time it takes is usually longer than people give it — often eight to ten minutes rather than two. Adding more spice to a dish where the first lot was never bloomed makes it dustier, not stronger, which matches what you found.

If you want to test which problem you have: take a spoonful of the flat curry, add a squeeze of lemon, taste. If it comes alive, it was acid. If it stays dull and powdery, it was the blooming.

Third cause, less common but worth checking because it is invisible: not enough fat, and the related habit of skimping on the onion stage.

Most of the aromatic compounds in spices are fat-soluble. In a dish where the fat has been reduced for health reasons, there is less to dissolve them into and less to carry them, and the result tastes thin in a way that no amount of spice fixes. The same goes for a base where the onions were softened for five minutes instead of properly browned — that browning is a large part of the depth people are missing when they say a curry tastes like it is missing something.

Neither is exciting advice. But if you are following recipes accurately and getting a consistently muted result, the two steps most likely to have been shortened are the onion base and the fat.