Low velocity can be caused by a number of things, many of which are dependent
on the gun's design.  I'll hit on all of them from stem to stern.

* CA.  Liquid usually produces vastly higher speeds than gas.  Some guns
  can't be turned down far enough if you run liquid into them.  Others
  seem to require liquid to meet 300fps.

* CA tank valve.  I have seen these stick and also get jammed where they will
  not open fully.  If the valve pin is removable, you might be able to fix
  it yourself, otherwise take it to your airsmith or just get a new valve
  or tank.

* CA tank pin.  If the pin is not being depressed fully, little (or NO) co2
  will be supplied to the gun.  Some guns have an adjustable depressor pin,
  and tanks with the thin pin can be depth-adjusted by turning the core usiing
  a small needle-nose pliers.  It's not recommended to back it out any
  farther than where the pin is level with the top of the valve.

* Valve spring.  the stronger the spring, the lower the velocity.  I have
  seen guns that someone has replaced the valve spring, and the new spring,
  though not really stronger, has more or wider turns, and cannot physically
  be compressed enogh to completely open the valve. (the spring's turns
  get completely compressed with eachother)

* valve cup seal.  Damage to the cup seal can make it stick or swell, or
  sometimes they come loose and lift from the cup seal cup.  These prevent
  the maximum airflow when the valve is open, and will reduce velocity.  I
  have also seen cases where an old cup seal has a deep groove in it from
  the valve seat and is sticking to the valve seat.  Replacement of the
  cup seal can be a requirement.

* valve pin o-rings.  Many semis have valves with small (usually clear)
  o-rings around the inner pin.  If these become dry or damaged, they can
  slow down the pin from moving and greatly reduce velocity.

* valve cap.  Some (usually semis) valves have a cap on them that is
  completely threaded on the outside, with no flaired end to press against
  the valve body and stop it.  (the Poison comes to mind)  Turning this
  cap in tends to compress the valve spring and lower velocity.  It can be
  in some cases turned in far enough to completely compress the valve
  spring, and reduce velocity to zero.

* valve velocity adjustent screw.  Some guns have a short wide screw that
  taps into the valve body from the side or back, and is usually accessable
  from the outside.  That's an adjustmet screw.  Most of them increase
  velocity when backed out.  Do not completely remove the screw, you will
  usually cause a massive air leak when firing.

* main spring.  Many pumps control velocity by changing the main spring.
  the stronger the spring, the more force the hammer hits the valve with,
  and the higher the velocity.  Most "velocity adjustable bolts" have an
  adjustable piston in them that changes the depth of the hole the main
  spring fits into.  Backing the piston out causes the main spring to be
  compressed more, and increases velocity.  If the piston is backed out
  too far, it can cause the main spring to be fully compressed before the
  sear reaches the bolt's lock ring, and the gun will not cock.

* hammer.  Damage or lack of oil (or introduction of grease) can cause the
  hammer to dramatically slow down after being released by the sear, and
  hit the valve pin slowly, causing a drop in velocity.  If it's a semi,
  check the o-ring on the hammer and make sure it's also lubricated and is
  not the wrong size (too large) or somehow other causing it to jam or
  drag.

* power tube.  many old pistols (nelspot 007 for instance) use a variable
  inner-diameter power tube to adjust velocity.  Thinner tubes produce
  lower velocity.  Make sure there's nothing blocking the power tube.

* flow-path adjustment bolt.  Similar to the nut in the valve, there can
  be a bolt protruding into the gas flow path leading to the ball, that
  is a velocity adjustment method.  Backing it out increases velocity.
  Again, do not remove it entirely.

* bolt o-ring.  If this is missing or does not seal up well, some gas
  meant to fire the ball will be escaping up the hopper, and lowering
  your velocity.

* barrel.  Long barrels usually lower velocity.  If all else fails,
  try a short barrel for an increase of maybe 30 fps.