If your Chives are looking a bit thin, it maybe time to split them up. For a large clump you can divide the clump using 2 forks, back to back, trim the tops down to 5cm or so, & re-plant. You probably need to do this every 3 or 4 years. Scruffy clumps you can probably pull apart by hand, trim the tops, & any wayward roots. Give them a bit of compost for a boost, & they like to be in full sun.