Quite a while ago, we had a couple of posts about felting, with a photo of two identical swatches -- one felted; the other, not. If you are new to felting, here's a quick checklist:
1. Use the right kind of yarn. Acrylic does not felt. Use an animal fiber, like wool or alpaca, where the washing instructions say "Hand Wash" for easiest felting. Wool/acrylic blends and superwash wools will felt/shrink eventually - socks made from sock yarn eventually shrink a bit. People have been known to run Plymouth Encore (75% acrylic, 25% wool) through the washer with nearly boiling water, and it did felt a little.
2. Make two identical swatches -- one to felt, one as a control so you can measure the percentage of shrinking. It's easy to skip this and say, "I'll take good measurements and calculate from there." It is more useful to have two swatches so you can really see the difference.
3. Is your washing machine a top-loader or a front-loader? Front-loaders are designed to be gentle on your washables, which is the opposite of what you want to do with felting. In this case, consider going to your local laundromat, or visit a good friend who has a top-loader you can use. You can also felt manually by vigorously agitating the piece in a pot of hot water.
4. If you ran your piece to be felted through the washer one time and not much happened, run it through again. Some yarns don't react much the first time around. You may need to run it through several times for the most felting/shrinking to happen. To maximize agitation, don't have too much water.
5. If you stitch in a tight gauge, the yarn may not have room to shrink much. The fabric may felt and get very solid, but it won't shrink unless there is room for it to get smaller. That is why felting patterns often call for big needles/hooks -- like a size 10 needle for worsted weight yarn, which is usually worked on size 7 or 8 needles. Consider bumping up your needle/hook size considerably in order to get a loose fabric that will shrink as well as felt.