Q.Begonia young leaves die off
I PURCHASED SOME SMALL BEGONIA PLANTS, POTTED THEM IN A COMPOST CONTAINING JOHN INNES NO 3. THEY STARTED OFF OK, BUT I AM FINDING NOW LOTS OF THE NEW SMALL LEAVES ARE DYING OFF LEAVING ME WITH JUST , MAINLY, THE ONE BIG ORIGINAL LEAF. GRATEFUL FOR ANY HELP AND SUGGESTIONS. REGARDS MR R HICKS

There are many types of Begonias and I'm not clear if this is an indoor plant or outdoors.
These links may help you.
https://www.gardeningknowhow.com/ornamental/flowers/begonia/begonias.htm
https://www.gardeningknowhow.com/ornamental/flowers/begonia/tuberous-begonia-care.htm
https://www.gardeningknowhow.com/ornamental/flowers/begonia/angel-wing-begonia-care.htm