Maybe your tension on cable is wrong! depending on the type of rear derailleur normally you have to begin your adjustment from the little gear and you have to adjust your cable tension correctly. But it always depend on what is your derailleur!!!
Sorry for my english
What drivetrain components are you using? What procedure are you following to adjust your derailleur? Parktool.com has very good instructions in their repair help section.
"...the people get the government they deserve..."
I am using SRAM X.7 front and rear derailleur, both newest model.
what instructions , I just mess around with H & L screws trying to figure it out, should I not be doing that?
"The eyes are the windows into the brain" -Timothy Leary
The high and low limit screws just determine the lateral range the derailleur can travel and once they're set the chain should rest nicely on the bottom cog without trying to go lower and also sit nicely at the top without trying to move up and over into the spokes.
After those are set you shouldn't ever have to mess with them unless you make radical changes to your bike's drive ie: changing an 8 to a 9 or vice versa.
The easiest way (for me) to quickly fix shifting problems is to reset the shifter cable by realeasing it, letting the derailleur move to it's default position, and then snugging up the cable and tightening the cable screw.
After that I test the system by running through the gearing...
I run the derailleur from bottom to top first to check that the limit screws are indeed set correctly and then work through the gears to make sure one click yields one gear change.
Coming up from the bottom a failure to shift is usually caused by insufficient cable tension and increasing the cable tension can be done at the rear of the derailleur (major adjustments) and fine tuning the derailleur is done at the shifter.
I also check the upshift to make sure the chain drops into the next cog as you want things working well in both directions and this is where he fine tuning generally happens.
All your adjustment screws (except the H and L screws) should be set near the midpoint so adjustments can be done both ways... turning them out 4 or 5 turns before the cable is attached or re-atached should do.
You should of course ensure that your housings and cables are clean and smooth running and that your deraiileur isn't bent
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what shifters are you using?
I just like riding my mountain bike.
As above release cable set H and L screw so RD does not cause chain to fall off.
Then set cable tension.
I am thinking you got a bent RD, have you crashed on the RD recently, look for scraps.