Most likely thing is the wrong bacteria / in the wrong place. Everything to do with the feeding - milk, creep and hay, as well as drinking water - should be scrupulously clean every time. Sometimes milk gets in the rumen, which is bad news. Usually it's a result of overfeeding (too much milk at one feed) or feeding technique (eg, head too vertical, milk doesn't run down oesophageal groove.)
Fresh, clean, soft hay should be available to all ruminants from before 7 days; it's the best thing for rumen development. Creep is less key, you don't really need them to be eating creep until you're getting close to weaning.