The reason is simple when I'm working with Canon RAW files LR produces wonderfull colours and makes great jpegs, sadly when I'm working with Nikon jpegs the same isn't true.
My work is mostly weddings, I also PP a lot of work for other photographers, those who supply Nikon jpeg (the majority) cause me lots of problems in trying to get good colours looking nice, the final images look "flat" and dull, and seem to have a lot less tonal range, for want of a better discription. I'm wondering if changing the colour space to something like SRGB will help (they are all saved out as SRGB, canon as well).[a href=\"index.php?act=findpost&pid=198741\"][{POST_SNAPBACK}][/a]
Are you dealing with JPEG files straight from Nikon cameras, or have the photographers done something to the files? If their cameras are set to generate AdobeRGB files, the EXIF data should indicate that, and Photoshop and Lightroom should both pick it up (Photoshop has the "Ignore EXIF profile tag" in the File Handling preferences) even though the files don't actually have the AdobeRGB profile embedded.
In the absence of that EXIF tag, if Lightroom doesn't find an embedded profile it will assume the files are in sRGB. If they were actually in AdobeRGB, the colours will definitely come out looking quite "flat".
If Lightroom was failing to recognise that EXIF tag in Nikon JPEGs, that would count as a bug. If the photographers are doing something silly to their files before you get them, that would also count as a bug (but not in Lightroom!).
When you open the files in Photoshop you get good colours: what profile is Photoshop assigning to the image? (I always have the Profile displayed in the Info dialog to keep track of things like this)
If it's AdobeRGB, when you save the file from Photoshop
and embed the profile then import that file into Lightroom the colours should look OK. But if you didn't embed the profile, Lightroom might assume the profile was meant to be sRGB (try Assigning sRGB to the images in Photoshop to see if the colours resemble the problems you're seeing in Lightroom). Of course, you shouldn't be saving files back to JPEG form if you're going to do extra work on them. But it might be a useful experiment to diagnose your problem.
Working from RAW files from the same cameras, do you get reasonable colours? You should.
Hopefully this helps!