Soft Caramel

I've been making sea salt caramels for a while now, and although I use a thermometer it still seems to be a bit hit and miss in terms of the 'bite'.  Obviously the ideal is fairly solid to start off with but quickly softening to chewy.  However, I started a batch yesterday which I cooked to 260 degrees as per the recipe  (and I've previously found if it's any hotter than that it goes really hard).  But having poured it into the tins, it just didn't set firm enough when it cooled down.  I therefore tipped it back into the maslin pan this morning and reheated it once again to just beyond 260.  Poured it back into the tins, and once again it refused to set beyond soft.  Anyone know what's gone wrong?