go off the boil [British]
to be less successful than in the past
Sandy Lyle says it pays to go back to the fundamentals when your game goes off the boil.
to become less strong or intense
If a relationship seems to be going off the boil, it is a good idea to appraise the situation.
to be performing very successfully
All three players are obviously on the boil at the moment in the Italian league.
Easy Learning Idioms Dictionary. Copyright © HarperCollins Publishers