There are many possible reasons
Your system may only support up to 2Gb (Win95, Win98original)
Your motherboard may only support up to 2Gb
You may have set it up wrongly - you must answer Y to the the first fdisk question about "Large hard drive support".
One or some of the above may apply to yoiu, but you will need to tell us more details about your system for suggestions how to rectify.