Slippage is a diagnosis, not an event
When a deal moves from this month to next, the visible event is a date change in the pipeline. The actual event happened weeks earlier: a step that had to occur did not occur, and nobody noticed because nobody had written down that the step existed. Understanding this is the whole game. If you treat slippage as something that happens at month end, your only response is pressure at month end, which is the least effective and most relationship damaging moment to apply it.
Treat it instead as a diagnosis with four common causes, and the response becomes specific. No genuine deadline on the buyer side. An approval step that was never mapped. A champion without the authority to drive a decision. Or a rep who has been reporting optimism as progress. Each of those has a different fix and a different early signal.