sounds more like your front mech is probably not quite right. also the best way to use your gears is to match largest 3 rear with smallest front, middle 3 to middle, smallest 3 to large chainring, assuming a 9 speed cassette.
Hi John, it would seem ulikely that the front mech is out and I can us all the lower gears from the middle ring - a bent tooth appears likely or a stiff chain link?
The only other thing I can think of is that your front mech has had a ding and is bent?
Do what XC Man sez. Tighten the cable and see if it makes a difference. Otherwise see if the the Front Mech has shifted slightly round. Is it absolutely square or is it twisted? Often happens after a crash and you just don't notice when you pick the bike up.
Other things that it could be but haven't been suggested.
1. Has the chain stretched?
2. Are you getting any chain suck? If yes see if the middle ring teeth have gone "hooky". If yes replace immediately before you wreck the chain and your rear cassette.
