Please help my Choisya Mexican Orange Blossom is sick

I'd look at the roots, see whats going on there, could be something in the soil, weed killer spilt there or something else?
asa last resort to save the plant I'd dig it up, wash the roots carefully then repot into new boughtcompost put it somewhere cool and shady, give it lots of tlc and be sure its kept moist not wet.
It may still die but worth a try
as to that spot then maybe try to see whats going on in the soil...

Any extra feeding won't help - I can't tell from the picture, but are the two which are suffering on slightly higher ground, that is, the ground slopes away from them towards the end where the others are healthy, particularly as the furthest one is the bushiest/biggest? That might mean they're not getting enough water. Otherwise, is there a power line or pipe or solid surface beneath where they're growing, something you've not discovered is there at a spade's depth, but further down?

I'd wonder about what's under the plants too. There's a spot in one of my borders where nothing lives longer than about 2 years. One of these days I'm going to dig down and replace the soil - that's the plan anyway!

Thanks very much for your suggestions!! I will dig up the sick plant (and maybe the slightly sick one as well) and put in pots today! The plants suffering are on lower ground - the ones doing well are higher up so maybe they are getting too much water? Regardless I might try and remove a lot of the soil in that spot and replace it. Thanks again!!!

I had three planted together, and one 'drowned' in a wet winter - it was on ground slightly lower than the other two.

Is there a reason for growing five in a row in that position?
They also seem to be quite close together so that if you are going to let them grow to full height and spread, you might find them getting rather congested.

We actually had 6 but the 6th one died. We wanted a hedge between us and our neighbours. Thanks for all the advice, I removed the 2 sick plants and put them into pots today. Fingers crossed they survive then I just need to work out the drainage issue. Thanks all again!!

It might not be a drainage issue - suggest you dig down a couple of spade's depths at least to see what you can see - sometimes power cables can affect a plant's growth, if you don't have overhead ones where you are. Also check the condition of the soil - does it look different from the stuff you dug up for the other plants in terms of smell, colour, etc. Maybe its an area where a previous owner parked a car or dumped oil, that type of thing.