MPPI,....I know that D.odora suffers from a number of viral diseases ...and some of these manifest themselves with yellowing of the leaves to some degree....although a better type of shrub has been bred to help overcome this.
My understanding is that once a viral infection has been caught by the daphne its outlook is extremely limited.
Lack of iron also causes yellowing in other plants....but the plant normally looks fairly healthy.

Daz

If not a virus MPPI, try using Camellia/Azaelea food on it. Mine picked up a treat after feeding it with this after I noticed it started to get yellow leaves.

My Daphne leaves tend to go a bit yellow over summer. I have found using epsom salts and some seasol has always helped the bush pick up.

Magnesium deficiency, which can be corrected by Epsom salts, causes yellowing of the leaf area between the veins - the veins remain green. There is probably more risk of a magnesium deficiency in a pot grown plant than one planted in the garden.

However, many gardeners swear by Epsom Salts. There is some research being done on its regular use in the garden (in America, where the soils are obviously different) but the jury is still out. What is known is that (a) Epsom salts is more effective as a foliar spray (dissolve in water), (b) while initial applications make leaf growth lusher, continued use seems to have less impact and leaf growth reverts to pre use condition, (c) it does seem to improve the size of fruits such as tomatoes and capsicums. It is also recommended that a nitrogen fertiliser is applied at the same time.

I have a huge daphne (which started life as a $2 half-dead from K-Mart) planted in a raised garden bed facing east and backed by an enormous myrtus communis. I can't remember when I last fertilised it and, because of drought and water restrictions over the last couple of years, it hasn't been getting much water either. But it generously covers itself with flowers and perfumes the front garden.

My daphne gets yellow leaves every year (but, dammit, can't remember when) which fall off - no apparent ill effects to the plant. I assume it's just shedding it's older leaves. A couple of my camellias do this as well - at the same time as they are putting out new growth.

I would suggest that if only some of the leaves are turning yellow, don't worry - it's probably just the plant's life cycle. If all the leaves are turning yellow, then there is a problem.

I'd suggest Camellia/Azalea food, a good deep watering, and top off with a thick layer of mulch.

Thank you all for the advice. I've tried Epsom salts and it seems to have improved the situation. This week I'll try it on a yellowing Camellia.
MPPI

G'Day . ok , Iam prepared to add some Epsom Salts to my Yellowing Daphne, but ,what would be the ideal mix ?
How much Salts to How much water

I've a new Daphne Eternal Fragrance that I'm trying to grow in subtropical Australia. It too has yellowing leaves (not just in the leaf margins, between veins - the WHOLE leaf is yellow in each case). I can't find any info on treating this on Daphne Eternal Fragrance plants, but I'm gonna assume a daphne is a daphne is a daphne. And then I'm gonna assume a daphne is a gardenia is a daphne is a gardenia - and treat it is I treat my gardenia which had this exact problem some months ago that I"ve totally fixed now.Both are in pots. Here's what i did with the gardenia - I read somewhere that yes they like epsom salts, but really really they like TONNES of water (and this is what causes the yellowing leaves more so than requiring the magnesium in epsom salts) BUT they dont want tonnes and tonnes of water while sitting on the ground. They want tonnes & tonnes of water, while sitting up on four little pebbles. Well you can imagine the good-natured "princess" talk my gardenia got as I placed those rocks under it, gave it epsom salts (not foliar) and then spent the next forever (5 months now) watering it daily. WEll, you will not believe the difference this made. Never a yellow leaf again, and never another application of epsom salts. I feed it my tea leaves occasionally and its leaves are green and lush and new growth seems to be sprouting forth daily (it's high summer here in Aus). So this is what i'll now do with my daphne (that i'm pretending is a gardenia) and i'll come back and let you know if this too fixes Daphne Eternal Fragrance's yellowing leaf woes.