Why won't my shower fully heat up?
Shower valves normally have a mechanism for adjusting the maximum water temperature under the handle.  You'll have to consult the shower valve's manual for specifics.  If you don't have the manual, here's a picture of a typical shower valve. The red part of the valve is the temperature safety.  It is located under the shower handle and you rotate it to adjust the temperature.  



Update:  This document seems to describe the procedure to adjust the temperature of your shower.  It looks relatively complex, but doable.