How old is the computer?
Old systems (4-7 years old) do not support the necessary power savings modes that power down attached peripherals. Period. Nothing you can do about it except either live with it or go wired. Older than 7 really a computer? snicker /forums/images/graemlins/grin.gif
Medium age systems 3-4 years old are transitional. They support the newest power savings modes, but some manufacturers didn't get it exactly right and some equipment works and other doesn't (sometimes it works right, sometimes it works with quirks and sometimes it doesn't work at all). There are usually firmware upgrades that you can download to flash onto the mainboard.
Newer systems (manufactured in the last 2 years) should work, if you have a system manufactured in the last 2 years, then there is either a software problem, or you are storing the mouse out of contact from the base unit BEFORE shutting the system down. Thus the base unit can not send a signal to the mouse to go to minimal power usage mode.
Keep in mind, my system ages are approximate so if you are close to one of those dividers it may still be affected.
If you bought a newer machine but bought a bargain basement system, it may also be using one of the older chipsets that had the power savings issues. Firmware upgrades are always worth a try.
Operating system also plays a role. Windows 95 isn't going to cut it. Windows 98 Second Edition is borderline, Windows ME, 2000, XP Home, and XP Professional all should have no issues with all current power savings modes.
Edit: If your computer is a mass produced from a well known manufacturer, go to their web site to get firmware upgrades based on model number. If you system is a home built, or custom system, get the firmware upgrades from the motherboard manufacturer.
Edit2: Not to leave out the obvious. But try it on another machine. It may just be defective. /forums/images/graemlins/blush.gif