Linear images are images which histogram have not been modified, the pixel values are original.
Stars are often not found when there are too many black pixels in the image, caused by misappropriate use of bias and darks, or too many white pixels in the image, causing stars to be saturated, caused by overexposure. Settings for star detection can be changed to try to improve it, there’s a shortcut button to access it from the registration tab, otherwise it’s in the PSF analysis window.