How are you viewing the jpg that makes it look flat? If you are viewing it outside of LR it may be that the viewing program is not color managed. Make sure that you export the image as jpg in sRGB color space.
Are you choosing sRGB as a color space when exporting?
If you choose Adobe RGB or ProPhoto RGB, colors will be desaturated when viewing the jpg in applications that are not color managed.
Use sRGB, unless you have a good reason to use Adobe RGB or ProPhoto RGB.
Hi Joe , Thank you for your quick response . I am viewing the jpeg in Microsoft Picture Manager and Flickr. It 's not always really noticeable , but for example if I shoot into the light with dew on the grass , the dew glistens in LRoom and I'm happy with the shot but when I convert it to jpeg ,it looks dull. I have changed the setting to sRGB - from Adobe 1998 ,but don't see much difference. Kind regards Alberto.