SAR backscatter is always dependent from various components, including sensor-specific characteristics (temperature, position,…), geometry (incidence and look angle) and atmospheric conditions (clouds still have a small impact).
If one image is different / darker than the others, you can
- apply thermal noise removal (some clarifications on that)
- antenna pattern removal
- calibrate to Sigma0 to level out sensor-specific differences
And, of course, you have to adjust the color scale of all images to the same range. Some outliers in single images could stretch the whole range which may cause the whole image to look darker than it is.