If you're using your remote control in IR mode (i.e. normal infrared mode), make sure you're pointing it directly at your TiVo box every time you change the channels.
If you're using your remote in RF mode (a mode that lets you change channels without having to point the remote at the receiver), start by checking the batteries, even if you've changed them recently; in RF mode, the remote consumes a lot more power. It must also be properly paired with your receiver (to do this, see section titled, How do I program my TiVo remote?), because once it's in RF mode, it can only be used with one device. If you aren't able to solve the problem, switch the remote back to IR mode (do this by pressing and holding the C and TiVo buttons simultaneously for 5 seconds). You will then need to point the remote toward the receiver to change the channels.
When pressing any button on the TiVo remote, the remote indicator light at the tip of the remote should blink, either amber or red. If the remote indicator light does not blink, you may want to try a new set of batteries in your TiVo remote, and ensure the batteries are inserted correctly.