Dave I am not sure you can eliminate Gall Midge and Thrips just cos the others around it don’t have it, I would very much like to know what Charlie says about that, I for sure don’t know about Thrips but I have read that Gall and other midges can be very local to 1 plant.
Here is a good read from Charlie on the Gall Midge
http://www.hiddenvalleyhibiscus.com/care/gallmidge.htm , you can see by the way they reproduce that it could easily be just one plant.
I have done quite a bit of research lately on this as I have a very similar problem to you, I have 3 cv’s which have near total bud drop.
Here is another read from a good source ‘The gall midge is a small fly that lays it eggs in flower buds. When the eggs hatch, tiny worm-like larvae emerge that damage the young bud, causing it to fall to the ground. The larvae then leave the bud and enter the ground to pupate and reappear as flies in about 3 weeks. Growers are successfully combatting the midge by using insecticides like Orthene and Cygon to kill them when they are in the bud and granular products Dursban and Diazinon or one of the replacement products for these two like Triazicide to kill them in the ground. Products containing imidacloprid are also extremely useful.’’
Dave I myself am now at the beginning of a full out attack on this pest [weather it Thrips or Midge I don’t know], I was very encouraged to see a small improvement when I used the Bayer 3-1 last month, this has in it imidacloprid, the problem was I ran out of Bayer but have now got hold of another product locally with imidacloprid in it and have started again, it does seem that you need to attack this problem if its bad with 2 products to be successful , 1 to spray the bud and the other a systemic.
I reckon I should know in about 18 days and I will let you know.