One of the most common mistakes I've seen is the backwards disc problem. There's only about 1/4" of depth difference but it's just enough to cause the exact problem you're describing. I've seen it at least 50 times since the mid-1950's.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!