Why are some of my courgettes doing this?

I’m taking no chances and making sure I manually pollinate but still it seems some just start to rot at the flower end whilst others are great, as you can see in the picture - healthy next to not healthy on the same plant ?!

Eggplants are self-fertile and shouldn't require hand pollinating. Have you had a lot of rain? Are they well drained? Blossom end rot in squashes is more commonly a pollination problem but in eggplants, peppers and tomatoes the issue may be a calcium uptake one. This has been discussed previously. If the soil is overly wet, or fluctuates between wet and dry at the time of fruit formation, the roots cannot uptake calcium which may lead to BER.

Ah thanks, now I have some possibilities to try and stop it happening again!
It’s probably calcium related as I have been very diligent in pollinating, whereas the weather is a constant battle between drought for a week, rain for a week…

t is caused by a lack of calcium in the fruit and the reasons are not always due to soil deficiency. ... Adding calcium to the soil is best done long before zucchini plants are grown. Use garden lime and epsom salts about 6-8 before planting your zucchini plants.