faint PCR bands

[rlee]

     Dear Netters,
     
     I am trying to screen transgenic mice by PCR.  I have primers for 
     different exons of the gene and use them to check the existence of 
     each exon.   The products for some PCR reactions come up faint.  I 
     checked for primer dimers and didn't find that to be a problem; I also 
     used aerosol tips to eliminated contamination of the DNA's during 
     preparation.  What is a possible explanation for this phenomenon?
     
     Thank you in advance.
     
     Rozanda