if they are not taking it, they don't need it. they are probably finding plenty at this time of the year. when you do your next check, see how much nectar they seem to be storing. you may not see a lot if they are using it quickly, but you should see some.
patties are pretty much a waste at this time of the year. they have to many choices out there.
if you are still concerned about stores, you can try adding a drop or two of lemongrass oil to your syrup, but honestly, the bees know what they need. :-)