who has conducted the clonetech supressive subtractive hybrid?

hi, I have a question to ask,
one friend of mine has conducted the clonetech supressive subtractive hybrid recently. and she did well all through the experiment, untill the last round of PCR, she gained no visible bands. But the PCR of the first round worked very well, a bright smeared area visible just as the protocol discribed. The second PCR is a nested one using 1st PCR as template. It should work based on logics, but she has tried several times and found no further improvement. Each step was set according to standard protocol. Can anyone give us suggestions? Thank you very much



Ph D. Xuebo Hu
Centre for molecular virology,
Institute of Microbiology,
Chinese Academy of Sciences