What would cause the starter to NOT engage the Flexplate?

[TIMSPEED]

OTHER than the flexplate being bent? The shop that put my flexplate in said it's NOT bent, so...I looked under the truck last night while my friend bumped the starter, and I WATCHED the pinion gear go ALL the way out and NOT engage the flexplate..so... Any ideas? The starter has a ALOT of shims on the outside mounting pad (like 3/4" worth of washers!) Should I replace the starter? (It's a lifetime warranty from Kragen's, but it DOES spin, so I'd have a hard time getting a new one...) Maybe replace the battery cables?
AHHHH! HELP!