Off the boil
Definition 1 of 3
Pro Tip 1/3Sports Usage
Often used in sports to describe a team or player losing their previous high performance level. 
After a series of losses, the champion has certainly gone off the boil.
Pro Tip 2/3Work Performance
Can describe someone's work performance becoming less effective or satisfactory. 
Ever since the merger, their customer service has really gone off the boil.
Pro Tip 3/3Colloquial Connotation
Carries a casual tone, apt for conversational use rather than formal writing. 
Our weekly meetings seem to have come off the boil lately.