If you mean the starter is not engaging the flywheel to turn the engine over, you might pull the starter and have a look at it.

#1 Make sure it is fully bolted in place, I have heard of starters held in w/ only one bolt or with loose bolts that do this.

#2. Check the condition of the bendix gear, make sure it is not worn out or packed full of grease and dirt.

#3 Check the condition of the copper contacts inside the starter solenoid. If worn unevenly, these can case the solenoid plunger to cock in the bore of the solenoid and get stuck before fully depressing the bendix gear to engage the flywheel: http://www.4crawler.com/4x4/CheapTricks/Starter.shtml