It's worth feeding anything that isn't performing and that you'd like to keep. You should wait for rain so the soil is moist and also clear any weeds or grass from the base as these compete for nutrients and moisture. Then you can add a generous layer of well-rotted garden compost and/or manure. You could also pour on ericaceous compost to help them - if they are indeed magnolias. Do not use mushroom compost as this is alkaline and will harm them.
That will help them over the winter. Next spring, give them a generous handful each of pelleted chicken manure or blood, fish and bone and some more mulch. This way you feed the soil and its organisms rather than giving a quick chemical fix and you'll end up with healthier plants. To encourage flowering, water with liquid rose or tomato food in spring but make sure you use rain water if the tap water is hard.