Best way to troubleshoot these is to start by uncoupling the shifter from the derailleur. Put some tension on the cable and count the clicks. As mentioned above, eight speeds = seven clicks. Tear into the shifter only if you're getting fewer than seven clean clicks.

An easy way to uncouple the system on most bikes is to shift into low, stop pedaling, then shift into high. This will put enough slack in the cable to allow you to pull a housing out of a slotted frame stop. That way you can leave all cable adjustments as is. For the front der, reverse the above gears.