One or two cancelled landings across a voyage is completely normal, and the expedition team almost always substitutes something rather than giving you the afternoon off. Usually that means a Zodiac cruise instead of going ashore, or moving to a different site entirely. Wind, ice and swell are the common causes.
Some cancellations are deliberate rather than forced. If guides scout a beach and find it too densely packed with wildlife, fur seals or nesting birds especially, they'll switch the group to a Zodiac cruise rather than push through the animals. That's the system working the way it's meant to.
The Falklands are the most cancellation-prone part of most itineraries, so if a specific Falklands site is the whole reason you're booking a trip, go in knowing that.