If you miss a scheduled port it's important to know a couple of things, firstly refuse any offer onboard of recompense, even if you accept a drink you will null any missed port cover you may have, secondly queue up at reception for a letter that outlines the detail of the missed port, these are already printed and are readily available but the cruise companies obviously don't like to make this known.
When you return submit the claim and it should be straight forward, I've claimed twice and it's been settled in less than a couple of weeks, both times we failed to get in Guernsey, on one occasion we paid £900 on a cruise for a family of 3 and received £450 in compensation so it's worth checking you are covered.