AlinMV,
I too had problems simliar to what youre experiencing
not long ago. That is not to say that I dont run into any stereo
imaging problems anymore but rather if I do I know where to
look.
I've read recently, that the brain actually uses the low level
information in a sound to determine its stereo position.
What that means to me is this..in order to get a real good
stereo image the low level information or the quiter parts
of the sound have to be better represented and not obscured.
Ways to do this...recording everything at 24 bit rates.
eliminate noise whenever possible. I was actually using a
home stereo receiver with my NS-10's and another pair of
home stereo speakers (JBL,s) as my monitoring system.
I switched over to
the Mackie HR824's....clean power and
better crossovers for sure....if you go the passive monitor
with power amp and mixer route..you will need to put some
money in the power amp portion of this scenario..in order
to get clean amplification. Use balanced cables everywhere
especially in the recording signal path.
The more tracks there are in the project..the more stereo imaging
is gonna be comprimised..again because of the accumulation of
noise from the individual tracks...
If you solo tracks and pan them and dont seem to get what
you think you be getting..check your software and make sure
you have the constant power selected as your panning option.
Also a little compression will help also..
In regards to the stereo/ mono recording...
You must remember that you have a stereo field ..which is the
space between speaker to speaker..and the stereo image which
which is two different representations of the same sound.
For a mono signal..all you can basicly do is move it around in the
stereo field...using the pan pot..if you took the same mono signal
created a dupe ..pan one hard left and the other hard right..the sound would still be centered..reason being that both sounds are
arriving at your ears at the same time.
A stereo image can be created by:
1. Using two microphones to record the same sound. The further
the mics are away from each other the more of the stereo effect
you will get. This due to the fact that the signal will arrive later to each side of the hearing.
The closer the mics are to each other the more chance of phasing..the symptom of this would
be a hollow sound due to the cancellation in the bass frequency
range.
2. Pseudostereo...this can be created by taking a mono signal
center it and EQ each side of the signal in a sort of opposite manner or just simply duping a mono sound pan it somewhere
left and treat it with a sorta opposite EQ than what you do the
one on the right...you compromise monocapadabilty with this method if not done correctly..
3. Take two mono sounds pan them opposite and change the
start times on one of the sound. I mention this because this can
be easily done in a DAW.
These are just a few..
Just another note on the low level information scheme..
this can be best personified by listening to a mastered
recording..because when those guys are done doing
there magic all of the low level stuff is clearly represented
because the crud has been eliminated or moved out of
the hearing range
Sorry if I rambled but Ive been there and I can feel yah..
Hope some bit or portions help!
I am struggling a little bit with the stereo imaging I am able (or rather unable) to accomplish with my home set up. I have difficulty when panning individual instruments, in distinguishing the location of the instrument. Although I obviously know which way I am panning, it seems to me I can hear the instrument equally in both channels, and the instrument has not moved. Rather the relative volumes in each channel may have changed slightly. 
