Voltage is only part of the picture - the one that makes all the difference is the amperage. If you are low on current the starter motor will struggle. So do the checks - starter motor and solenoid, ground leads, starter wire connection and main power lead to the starter motor. One by itself may not be enough to cause a starter fault but a combination of problems definitely will. The starter motor solenoids can get gummed up as well. I also read on another members' thread recently about the bearings in the starter becoming so worn the armature no longer lined up in its housing correctly, resulting in poor engagement.