I have a problem where an image does not have enouph color information for me to color correct it properly for print. I know there is a setting that can be changed, that when the image is changed from RGB to CMYK it will automaticlly assign higher CMYK values to the image. I just can't remember how!
It sounds like the RGB colours in your image are outside of the CMYK gamut. My suggestion would be to use 'Hue and saturation' to bring these RGB values closer towards the CMYK gamut before converting. In your 'info palette' change you colour values diplayed to CMYK and place the cursor over the image, some of the values shoud have a ! symbol next to them which means that these colours cannot be represented in CMYK. Try using Hue and Saturation and reduce the 'saturation' of the image until the ! symbols no longer display next to their CMYK values. Then OK the adjustmens (or merge the adjustment layer) and convert the image to CMYK. If you want to boost back up the colours while in CMYK use Hue and Saturation again and increase the saturation but be careful because the slightest increase can change the image drastically.