Suggestion #1: Crop the photo hard to the edge of the flower on 3 sides and leave a little more space on the lower edge of the flower.
Suggestion #2: It may be interesting to try this. Could you somehow fix a smallish mirror against the wall (blue tack would help) so that it reflects more of the under side and back of the hoya? Then photograph the flower and its reflection.
Suggestion #4: Could you take the photograph lying against the wall and beneath the flower? If so, try using flash as well, experimenting with flash at about half power or less.
Suggestion #5: Come to N.Z. They grow all over the place and a good point of view would be easier to arrange.
Good Luck! PAUL