Everything slips. What matters is which parts can absorb it and which cannot, so plan the buffers deliberately.
A timeline built on the assumption that nothing runs late is not a plan, it is a wish. Something always goes long, usually the part with the most people in it, and the question is only where the lost time comes back.
Some blocks absorb delay for free. Drinks receptions, photographs and the gap between courses can all lose fifteen minutes without anyone noticing. Others cannot: anything tied to sunset, anything the kitchen has already started, and anything a supplier is leaving at a fixed hour.
So put the buffers next to the fixed points, not spread evenly across the day. Twenty minutes of slack before the speeches is worth an hour scattered across the afternoon.