Two herb gardens, different outcomes

I've planted two herb gardens with similar herbs.

They've both got sage in the middle, although the purple sage in the top photo is much bigger because it was an existing plant from a pot and not grown from seed. They've both also been seeded with parsley at the front, flat leaf in the top picture and curly in the bottom picture, but only one has grown. Then the bottom picture also has two types of oregano either side. None of it grew and the small scrap of oregano on the right is actually some I grew from seed in a pot last year. In fairness, the chives are also from a pot and not from seeds. I planted the rest of the segment with seeds, but they haven't grown at all yet. And my attempts to grow thyme round the edges are going equally badly except for the one I bought in a pot from B&Q. I'm beginning to see a pattern here or most of my herb seeds failing and existing plants from pots doing OK.
Given how the slugs suddenly started devastating my chervil (rear section of the top photo), it made me think maybe they'd been eating my seedlings in the bare earth plot as soon as they came up. So, much as I try and do without slug pellets, there are now slug pellets down just to give the seeds I've now freshly planted a chance.
The garden as a whole is thriving though.