[Leica] Difficulty Focusing Summicron 90mm

[Jim Gallagher <gallagherjt@earthlink.net>]

I'm a novice looking for some advice regarding a problem I have been 
having  in low light situations. Specifically, I have been using my M2 
with a Summicron 90mm to take photographs of performers participating in 
a local youth orchestra. Almost without fail the performers I am focusing 
on are slightly out of focus while those one row back are clear and 
sharp. I'm using a monopod, Tri-x 400, shooting either wide open or at 
2.8 and about 1/60th of a second shutter speed.The theatre is dark and 
the stage is well lit. I usually use the violin bow or ear of a performer 
as my focusing target.
When I switch to my 35mm Summicron the results are almost always sharp, 
no doubt due to the larger depth of field. In addition, when I use the  
90mm lens outdoors at higher f stops the results are usually sharp. 
 What might explain the fact that my focusing target is slightly out of 
focus but three feet to the rear is sharp and clear? Shoul I be focusing 
three feet in front of the target or do I need to get my camera adjusted?
Thanks for any help!
Jim Gallagher
Saratoga Springs, NY