Are you using anything other than the stock charger? Many aftermarket chargers have slow charging rates. If you are plugging it in to a car charger, they are notorious for charging slowly. If you replaced the cable from the charger to the phone with anything other than the original, it may offer enough resistance to slow the charge.
If your phone is using power at a higher rate (running apps, etc), it will charge slower.
Have you installed any apps that might interfere with battery usage, charging or reporting? Are you running stock or have you rooted and installed a custom ROM or kernel?
Did it use to charge faster? What has changed as far as environment/equipment/software/usage since then?
There are many reasons a phone can charge slowly. It would help to have more information. If you would like to have a discussion about this problem, I suggest registering (see link below in my signature) so you can reply to threads including this one. That way we may be better able to help you.