A photo would be very useful, otherwise, is the shrub in the ground and how long have you had it? Are all the leaves going yellow or just some of them? When you say they're going yellow, do you mean actually yellow all over, or a yellowy shade of green, or mottled yellow and green?

30years old some leaves have turned yellow all over

I don't know why this would happen on such an established plant.....you've not fed it with anything alkaline? Try a feed with sequestrate ...Helped mine when they were sick.

if its only a few then it is the natural shedding of the dead /old leaves. Most evergreens shed the odd leaf now and again. if it is more substantial then it is likely be due to lack of nutrients. give it a feed of sequestrated iron or as a quick remedy a dose of Epsom salts/magnesium sulphate.

but a photo would be good.

That is what I would have said,..natural shedding,if it's only the odd few.I took some of mine off yesterday,but the rest are ok..Ericaceous soil is used mainly for acid loving plants,such as Camellias,Rhododendron,Azaleas,Pieris etc..so a good mulch with that would be beneficial,as well as a feed as suggested above..

Thank you all for info will give your advise a go ?