Why don't Sunflower Seeds Compost!!!

Now dont get me wrong, I love sunflowers in all their many varieties. And I dont even mind them self sowing in the garden, but in the council delivered compost there are literally 1000's popping up all over the place. But why haven't they composted down - I thought in comercially produced compost it got rid of all of seeds etc. I am also suspecting that my newly aquired bind weed might be from the compost too. As i have never had bind weed on my allotment before.

But not a single one I purposely planted down the allotment has grown, in this rubbish weather. and the ones in pots in the garden, the slugs have chewed through the stems!! No even just a nibble on the leaves - the whole plant is gone!!!