There could be several things wrong.  Most likely is that the lock is loaded
with dirt/grease, etc.  WD40 is about the worst thing you can put in your
locks,  It reacts with the grease VW used and gums it up to a very thick
consistency.  I would suggest cleaning the lock first.  To do so, remove
from the vehicle.  Liberally spral carburetor cleaner into the lock both
through the keyhole and in the rear of the assembly.  When I overhaul the
locks, I dismantle them to clean.  If you decide to do so, you must remove
the tailpiece from the lock.  It is held on by a small roll pin which must
be driven out.  the lock will then slide out the front.  BE careful that you
don't lose the wafers and springs.  If you keep a key in place, these parts
can't be lost, but it makes cleaning more difficult.  After cleaning both
the cylinder and the handle (the hole where the lock resides), it can be
lubed with a teflon or silicone based lubricant.  Tri-Flow is fairly readily
available at hatrdware stores.  Reassemble and it should work.  It's also
possible that your key could be very worn.  A new key cut to code whould
solve that.  If you don't have the code, it's on the tailpiece of the lock,
but is frequently covered by corrosion.  it's either 4 numbers or the letter
"N" and 4 numbers.  I can generate a new key from that.  Don't just copy
what you have; that just makes a worn out key on a new blank! Feel free to
email me should you need further information.  Good luck
Steve Sandlin
www.steveslockshop.com
ssandlin@lcc.net
'84 Wolfsburg Westy
'79 Transporter
'00 New Beetle