There was a plan to liberate them as early as 1943, called Operation Constellation. Drawn up by Lord Louis Mountbatten, based on aerial reconnaissance.

The problem was that, as it stood, the Channel Islands weren't actually much of a strategic advantage. Obviously if the Luftwaffe were to establish a bombing base there, that would change, but until 1945 the Germans hadn't made much of an advantage of the occupied islands. Really, the only purpose they had served for the Germans, was as a slight bloodied nose for the British.

Mountbatten observed that while the islands could be retaken by force, they were also heavily fortified, and it would require a fairly large landing force to retake these strategically unimportant islands.

The other consideration was the civilian population. They hadn't fared too badly under the Germans, there were no reports of abuses or reprisals, but attempting an invasion might put their lives at risk. Certainly their lives would be at risk if a battle broke out.*Mountbatten also noted that either aerial or naval bombardment would be necessary in order to break the islands defences, again at a potentially high civilian loss.

Mountbatten's plan was eventually rejected on the grounds that the cost would likely be far too high, and that the sensible option was to have them peacefully returned as part of Germany's unconditional surrender.

I'm afraid the only source I have for this is Lord Louis Mountbatten's biography, "The Life and Times of Lord Mountbatten", John Terraine, 1968.

There is also a wikipedia entry for Operation Constellation, and most of it seems to back up what I've read in Mountbattens biography, for what it's worth:

https://en.wikipedia.org/wiki/Operation_Constellation

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