1. They had the opportunity to make a lot of money, but they didn’t act quickly enough and the chance slipped through their fingers.
2. When Mike asked Irene to marry him, she told him she wanted time to think it over. But she waited too long and Mike married someone else. She let him slip through her fingers.

Synonym: miss the boat. The expression suggests that the thing or person one wants to keep is slippery or hard to hold on to, like water or sand. Despite the fact that one tries to grasp it, it disappears between one’s fingers.