I hate to sound flippant, but you've already figured it out: "What's going on" is that "Island Hunters" is only shown occasionally.
It was never a fixture of the HGTV schedule. Each season only has about 12 or 15 half-hour episodes, and there are often long breaks between seasons (at least by reality TV standards).
In fact, it's barely a show on its own. "Island Hunters" is better understood as a sort of sub-series within the sprawling Hunters franchise. It started with "House Hunters," but has since expanded to 18 different shows dealing with people's (very) specific housing needs.
They range from humble searches for a rental house and compatible roommate to more grand desires for castles or, in this case, private islands.
"Island Hunters" is actually one of the more stable shows in the franchise. It's run for five seasons so far (the aforementioned "Roommate Hunters" only got one), and as far as anyone knows it's still going. The fifth season just aired this past fall so at this point there's no reason to believe there won't be a sixth.
As for the fact that it only airs occasionally at the moment, that seems to be part of the larger Hunters plan. All 18 different shows air in reruns all over the HGTV schedule, filling schedule gaps where needed.
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