Are these home spawned or store bought or bought from a breeder.
Since they have otherwise been fine for over a year and the buoyancy issue is new-A few things come to mind-aging, too cool/dry air over the water, poor quality food, bacterial infection.
Be sure and get the humidity and heat up over the water-you want to see condensation buildup on the walls-I use plastic veggie wrap-but anything will work. Lower the water level to limit struggling and no water movement. Start Epsom salt 3tsp/gal and tannin treatment.
Premix your treatment water to use for 50% water changes every other day for 1 week-then every 3rd day for 1 week. Then re-eval..