If the drivers/pads aren't angled you can try switching from wearing them L/R to R/L (wearing them the wrong side) to see if there's any audible difference. On Mono. This can give you an idea if there is something physically wrong about the audio component. Assuming you've already checked with another headphone to see if it's not your ears or the source.
About the fit, typically speaking a bad fit will not affect the volume to such an extent, but the sound would likely lack bass if anything. The best way IMO to test if it's a fit problem is to either
1. Press them down against your ears so make sure there is a seal.
2. Try adjusting the position based on the fit/comfort of the side where everything seems right, and then do the same for the side that feels weaker (trying one side at a time, not trying to get perfect fit on both sides).