There can be several things causing your problem, ranging from a key too
worn to severe corrosion.  It's possible that a spring may be stuck or
broken or a tumbler frozen in the lock.  Most of these problems can be
repaired.  Feel free to email for more discussion at
steve@steveslockshop.com.
Steve Sandlin, registered Locksmith
Steve's Lock Shoppe
http://www.steveslockshop.com
'84 Wolfsburg Westy
'79 Transporter
'00 New Beetle