Daphne Odora only half flowering

yellowing of the leaves could be one of a few things. Lack of magnesium or the other minerals needed for healthy chlorophyll [green pigment], the natural death of the older leaves but if its most of them thats unlikely. Is the ground waterlogged more than usual as that is also a symptom.
As to why one side is bushier than the other that could be down to the amount of sun it gets, when it was last pruned etc.

Thank you, SBG. It is in an area that only gets a little sun and could be wetter than usual from too much winter rain this year. The shrub only has two main branches so should I cut the yellowing one off or leave it?

This winter I seem to have more problems than usual, no doubt due to the change in weather patterns which I suppose we should blame on global warming. However when you look back in history there seems to be a pattern of climate change so I’m not totally convinced that it’s ALL man’s doing!

No don't prune it back just leave it and perhaps give it a dose of Epsom salts. teaspoon in a pit of water. If the soil is very wet then lightly fork around the shrub too, 2foot away from the main stem to reduce damage to the roots. push the fork in and just rock it back and forth to loosen it. no need to lift it and turn it over. This will help drainage and also allow air/oxygen into the soil structure.

My Daphne is only flowering on the lower, more sheltered stems. I think the upper part of the plant suffered from very cold winds and possibly frost. It is a shame because all the flowers are hidden away.