Remember the importance of "the cone" and that it's the physics principal governing
flintknapping and the mechanics of flake removal.  If you knap with your biface on your thigh, try
moving it to the outside of your leg, but still keep the bottom of the biface tight to your thigh.  Now
instead of your edge being parellel to your leg, it's now pointing up in the air.  What this does is
bring more of your biface surface in line with the cone edge angle, therefore resulting in longer
flakes.
    If you are a hand knapper, just change the angle of the biface striking edge so that it's tilted up
more.  It will achieve the same effect as when the lap knapper moves the biface to the outside of the
leg.  The more you tilt the biface striking edge up, the longer the resulting flake will be.
    Another problem that I've seen and may be your problem too is that you are either not hitting
the biface hard enough or you are stopping the swing of your billet at the edge of your biface instead
of following through.  Like in karate, you don't aim to hit the biface, but instead swing through it
toward the floor.  Your swing should always be the same, it is your biface that changes the angle so
as to manipulate longer and larger flakes.
Answer
Here we see a picture of the biface on top of the thigh.  A
billet strike in this position will produce a short steep flake
on the striking edge.  You will never produce long flakes
from this biface position.
Striking edge
Top of thigh
Top of thigh
Outside of thigh
Outside of thigh
Striking edge
Here we see a picture of the biface moved toward the
outside of the leg with the striking edge angle more at an
upward angle.  Now with the proper prepared striking
platform, a longer flake should be produced
Knapping on the leg
Free hand knapping
Here we see a picture of the biface held at an angle
parallel to the knappers leg.  A strike at this angle will
only produce a short steep flake
Here we see a picture of the biface held at an angle that is
quite tilted by turning the wrist outward.  This angle will
result in a long flake, and even an overshot flake at this
aggressive of an angle.  I would not advise an angle this
tilted at this late in the example bifaces stage.  I used this
kind of angle to demonstrate the tilting in an obvious way