There maybe many reasons result in this phenomenon. But mostly will be these causes:
1.Not changing the planting media for a long time. This will result in a lack of phosphorus fertilizer and other elements.
2.A bad root system will cause poor nutrition, therefore, the plant will not bloom.
3.Being short of water for a long time.
To improve the status and make the adult plants bloom again, you should:
1.Change the media termly(better 2 times per year, one in spring, the other in autumn) and add some fertilizers which have phosphorus, e.g. Bone meal, fish scale, etc.
2.Mix a bit more sand or small gravel into the planting media to foster the roots and make the roots renew as soon as possible.
3.Dip the pot in the water for a certain long time, do not water the dry medium directly because that will cause the water pour out from the cracks and the roots still can not draw the water.