Answer: Based on your description I suspect it is slugs, they just love this plant. They are nocturnal so you may not see them, but you might see their shiny trails left behind. You can stop them by sprinkling diatomaceous earth (the horticultural type not the kind sold for swimming pools) around the plant or using one of the newer slug baits formulated with iron phosphate. (These are much safer around children, pets and animals than the older metaldehyde bait.) You can also hand pick them at night by flashlight and toss them into soapy water to drown. The plants should recover. I should mention however that these plants do better in an evenly moist soil than in a very dry one. You will need to keep their roots evenly moist like a wrung out sponge (not sopping wet) at least while they become established. Since you have slugs it is better to water in the morning and water deeply less often than to sprinkle lightly every day. Also, make sure the grass clippings do not mat and become an impenetrable layer. If you can mix in a little bit of other mulch eg shredded bark to prevent that it would be great. Enjoy your lamium!
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