Any change in light or water flow?
Bubble corals do best with low to moderate flow which should be just enough to keep the bubbles moving slightly but ultimately, aim for gentle water movement. Anything more can actually damage this coral. This flow is needed for bubble to remove algae and debris as well as for feeding best at night as they use their sweeper tentacles to help catch food ditributed by this current.
Lower light keeps them happy and up to 150 PAR.
They require nitrates, phosphates, calcium, magnesium, and stable alkalinity especially Mag, alk and CA. regular water changes will help maintain these levels.
Lastly is feeding mentioned before. Good foods are : small pieces of shrimp, crab, brine shrimp, mysis shrimp, and small plankton