The most likely cause of the yellowing of the leaves on your clivia plant is overwatering. Clivias should be watered only when they need it. Let it dry out to about 50% between waterings.

Another possibility is being in too sunny a location. Noon and afternoon sun is too strong. If you are moving a house plant outdoors for the summer, place it in  a semi-shaded location.

Lack of nutrition may be another cause. Feed the plant  with a general fertilizer (diluted 50%) every two weeks from April through September.

Remember that it is natural for old leaves to die as younger ones grow.

Finally, clivias need a rest period before flowering (say between October and January). During this period water even less. Keeping the plants in a cool location will also help to initiate blooming. 

Courtesy of NYBG Plant Information Service