Sometimes it’s the rhythm, the semantics, the grammar. Reading and reading and reading again will point out problem areas.
From A Bottle of Beer:
“He thought that it slept, unconscious in the blistering noon sun. So the mouse scurried forward, first a few paces, then a foot and another. The snake did not move.
Cocky and no longer cautious, the mouse darted in front of its nose, squeaked a curse in gleeful retreat, and then its whole world turned black.”
A simple change:
“He thought that it slept, unconscious in the blistering noon sun. So the mouse scurried forward, first a few paces, then a foot and another. The snake did not move.
Cocky and no longer cautious, the mouse darted in front of its nose, squeaked a curse in gleeful retreat, just as its whole world turned black.”
Last sentence, “and then” has been changed to “just as” because “then” is a telling word, answering the action question of “then what happened?” but also because it’s a stalling phrase. “And then” indicates happening after something that already has happened. “Just as” indicates simultaneous action.