The basic rule
Communicate the delay as soon as you see it coming, not at the deadline you already know won't be met. A client warned a week ahead can replan; one told on the day of the deadline can't.
What a good message includes
- The real reason, no vague excuses like "some issues came up".
- The new estimated deadline, as realistic as possible.
- What you're doing differently so it doesn't happen again, if relevant.
Trust is built in the hard moments
A client who sees you being transparent even when the news isn't good ends up trusting you more long-term than one you kept promising only good news that never came true.
Bottom line
Delays happen in every business. What actually matters is how fast and how honestly you communicate them, not whether you manage to avoid them completely, always.