I am looking for few nouns describing "a situation when one does too many things but achieve nothing important or useful".
closed as not a real question by RegDwigнt♦ Jan 23 '13 at 13:04
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If there's some resource shared between many targets/users in a way that each of them is short on it, you spread [the resource] thin over too many [targets].
In this case You spread your time thin over too many tasks.
You're looking for the expression spinning your wheels.
Example: They're on the grind spinning their wheels.
You can also invent your own expressions such as:
Circling around in circles (suggesting no movement or direction inspite of activity) They're pushing too hard, a door that needs pulling. Or they're pulling what needs pushing. They're pushing a brick wall. They're busy chewing the stones. They're busy chewing the rocks. They're taking one step forward and two steps back. They're trying to fleece leather out of a sheep. They're working hard counting the hairs on their own heads. They're making about turns to get there.
It may not be exactly what you want, but a word in roughly the same semantic area is counterproductive.