It's hard to say whether your wife isn't nude-ing up because of any hang-ups over her body, or because she has been raised to think that it's inappropriate to be nude other than in the bedroom since I don't know her personally. Either way, you can't really force the issue if she plain isn't comfortable with the idea.

If she is indeed self-conscious about her body then you need to keep encouraging her with flattery and reassurance that you love how she looks. Perhaps buy her some nice underwear (and I don't mean lingerie necessarily) that she can slip into on days when you're hanging out at home. Cute tank-tops and shorts perhaps? How about also treating her to a sexy but tasteful professional nude photo shoot? Perhaps she doesn't know how hot she can look and this could be a good step to getting her comfortable in her own skin.

If your partner simply doesn't enjoy being nude because the naked form just isn't 'right' for her, I'm not sure what you can do to convince her otherwise. How we are raised can determine how we act as adults. Perhaps a combination of the above suggestions may also help in this scenario.

Whatever you do, don't badger your wife too much about it. She'll go nude when she's good and ready.