Sometimes, I've had batches ferment all the way to dryness, 0.998, in less than a week. The yeast used can make a big difference sometimes!
I gather that you did not use cultured yeast to ferment any of the batches of cider, but allowed wild yeast to do this job.
I would compare the current SG readings of the slow batches to the current SG readings of the other batches.
Depending on wild yeast may be problematic sometimes, especially when the must has been boosted with so much sugar.
If the SG tests high on the suspected batches, I would first try adding yeast nutrient.
If that didn't do the trick, I'd re-pitch the yeast with a packet of some of the cultured stuff of your choice.