It was said that a build is delayed and then a manager asked why the build was slipping.
I have looked up the OALD for the verb slip and didn't find any evidence that 'slipping' has the meaning of 'delaying'.
Could you please help to find out what is the meaning of that word.
Without further context, I think that in this case "slipping" means "slipping/lagging/falling behind" the deadlines or expected rate of progress set for the project.
not a teacher