need help on megeprimer method of PCR mutagenesis

[yqj]

I am now using megaprimer PCr method to do site-directed mutagenesis.
I use three primers---one internal primer with the desired mutation built in
and two flank primers. The problem is that. When I use PCR1 product as primer 
(the megaprimer) to run the 2nd PCR---PCR2, the pcr product yield is very low.
Can anybody who has megaprimer pcr experience give me some idea? 

Deqi Zhang

dqzhang at andromeda.rutgers.edu