My assumption would be that your nitrates are very high - that will cause shrimp deaths and usually only the youngest shrimps survive, but they won't breed or grow well under those conditions. A 10% water change is really not much at all - I do a 30-50% change once a week depending on my water tests or my nitrates start to creep up (I try to keep mine <10ppm, with <5ppm being better). You really do need a test kit of some kind, but failing that most fish stores (even Petsmart and Petco) will test your water for free if you take it in, so there's really no excuse NOT to know your water parameters.
Algae wafers aren't complete food for shrimp - shrimp need more mineral content than they provide. I only feed my shrimp every 2nd or 3rd day (2-3 times a week) a small amount of either Jake's Om Nom or Mosura shrimp food. Daily feeding isn't necessary in an established tank.. when they don't have "food" they will spend their time grazing on biofilm and algae. I know some people feed theirs even less often - I've even heard once a week.