The greenhouse may be ventilated but the heat inside might be too high. High heat will cause yellowtop

often due to high temps and direct sunlight on them. It happen sto mine and I just cut that bit off before eating them. Try shading the glass next year.

I guess this happened because of the sun as it was mentioned above

Thanks for all your answers but I've done everthing you have recommended I'll put it down to climate change .