If I remember rightly you need an exact bearing to reach the island, and as far as we know, I don't think Jack knows it, or at least, doesn't remember it. At best he knows it's near Figi. Also, Ben was hiding the island's location from Widmore for ages via The Looking Glass (jamming signals I think?). Aside from this, the island seems to stop people doing things even when they've left it (e.g. Michael's attempted suicide), so maybe it's somehow stopping Jack from returning?
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