Can anyone help me out please.There is whitish stuff under some leaves and eventually the leaves go yellow and drop.Whatever it is (?)it responded to insecticide soap.I just sprayed it a while back and it got better.I only gave it 2 or 3 sprays so possibly not enough.Now its back so I am trying to figure out what I am dealing with to treat it properly.

Please help me out because I have two of lovely star jasmine and would like to see it healthy and with flowers.It has lots of buds but also lots of whitish stuff under leaves.It makes the leaves go black and a bit sticky then yellow and then they just drop off.Sorry that's the best description I can give here.One day I shall figure out how to post a pic here so it will be seen easier.

Many thanks and I shall wait for help because I am loosing this battle on my own..