cmyk question

why doesnt work some filters in cmyk and lab documents in photoshop?

Because some are designed to only work in RGB mode. Many filters don't work in indexed or greyscale modes either.

Mostly all default filters in photoshop are designed specifically for RGB color model, loosely called "light colors", opposing to CMYK "pigment colors" which are those that monitors use.
What you can do to work with CMYK is convert to RGB, apply effects, and then re-convert to CMYK.
You will need to make some minor adjustments.

yes rgb has a colour range that cmyk cannot reproduce, so yeah just convert it to rgb then apply, depending what it is ill usually work in rgb till the file is ready to print then convert it, not always best but it does allow you to play with the image a bit more.