yellowing and leaf drop is normal - but is sped-up by inconsistent watering and the amount of water received ....... adeniums hate change - and that's what is going on
my plants do the same thing - early spring until mid summer we have weekly rains, and this keeps plants watered .....this is followed by drought conditions, consistent watering drops off or stops... I may skip watering if there is rain forecast in the next few days - rain never comes and yellowing leaves and drop off is the result ...
step up watering just a tad
it is better for adenium health and growth to be on the dry side, rather than constantly wet