[LAU] Can't get the stereo imaging from the speakers

Hello, I got speaker stands from the Yamaha HS5 powered speakers. So they are at ear level on each side of the desk. This results in hearing the music not as it comes from the speakers, but directly from the center of the monitor. Which is nice. One would think the audio comes from the monitor. To this the HS8 subwoofer sits on the ground under the desk to the left. To the left is also some 12x12 audio foam tacked to the wall, for guitar recording. The problem with this is that when a track is panned completely to one side only its volume decreases. The audio of that track does not move to the side. I have to use the headphones many times in the mixing to get the stereo imaging. Which is OK, as I like to get the headphones point of hearing, but it'd be nice to hear the stereo effect through the speakers also. Any advice on how to use speakers so that it's possible to hear the stereo imaging ? Cheers.