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Have you tried snaking out the bowl with an auger? If you did or you absolutely sure the toilet has no blockages then this is most likely a venting issue. A toilet that's on a drain line that isn't vented properly won't flush. The water will just swirl in the bowl.
Well it might be worthwhile to run a closet auger through the bowl just to be sure since finding a blocked vent (assuming the drain is vented at all) will be a lot more challenging.