Mottled leaves on petunia, going brown

[Melassa]

Hi, what could it be? My petunias are sprouting now green shoots but as soon as the leaves get a decent size the become all mottled the go brown/yellow and die off. I read somewhere it could be a lack of nitrogen so I’ve been adding coffee grounds, but it hasn’t stopped the problem. The flowers that come up are healthy but lower down the plant there is gradual die back as the plant grows. The stalks have ended up quite bald but the green, leafy tips. I have another petunia that this hasn’t happened to, so I am mystified.

It could be a virus. Tobacco mosaic virus (TMV) can affect petunias as they are in the same plant family, although this is far more prevalent in the USA.
If it is a virus, the affected plants should be destroyed, not in the compost bin, but wrapped and disposed of in the household waste.
However, a photo would be a great help!

Thanks @NanTheWiser. Sorry, photo here. I got a lot of new shoots this year, so I thought it was a nutrient deficiency as opposed to a virus or a fungus (I did try anti fungal treatment too and neem oil).