1)Looking at that picture I'd say the ones that are showing are biggest in the middle & getting gradually smaller towards the edges. If I have seen that correctly, is the middle wetter/warmer/not as solid/etc?
2)Do you have voles? One year we had quite a few different things missing at the edges of beds. A neighbour said they had more voles than usual so we put some live catch traps out & , yes, voles it was. Don't know why but they started their munching at the edges!