PCR problem for Site directed mutagenesis

[Panapat Uawithya]

Hi,
 I try to make a mutagenesis follow quickchange protocal. I have succeed
for several mutation except one mutation. My primer can bnd speccific to
the template when using Vent amplified a short fragment (< 1 kb)
When I try to use these pair of primer to amplify the whole plasmid with
vent DNA polymerase I got nothing (Pfu cause non specific binding, I
don't understand why).  When I try to used those short amplified
fragment from previous experiment with Pfu I still got nothing except
smear band from the expect size down the primer dimer.
 how could I solved this problem?

Regards,
Panapat Uawithya,
g4037881 at student.mahidol.ac.th