[pwgun]

last year had excellent marrow courg, butternut,  but this year i cannot get the plants to grow, and i have tried about 80 or so, slugs had first big batch, and others go yellow, skinny at soil level, or just die.
i have got 2 plants going ok and two more looking promising.

what is going on?

[Trillium]

Not quite sure but a lot of people are having the same problems this year. Could possibly be something from last year's very wet summer affecting things this year, or possibly poor seed crop. Though a seller may get seeds, they may not be of a vigorous nature for this year.
Suggest you try a different brand of starter mix, fresh seeds if you can find them and try a different spot in the garden/plot if you can. At this time of year the seeds should sprout very quickly. In the fall 'tent' the plants in a bit to let the crops ripen.

[mashauk]

I tried mine on my flat roof where nothing had ever been eaten by slugs/snails - before now!  I can just picture them getting a whiff of plants up there from the ground where there's more than enough to eat anyway, clambering up there (it's a good few feet) during the night, stuffing themselves silly and slithering down again to hide in the morning.

[Trillium]

Last year's very wet summer would have produced copious amounts of slugs just waiting for this year's crops.