I think that your sample maybe could be degradeted. If you use this sample several times or is a old sample the DNA became to degradate to several cycles of freezing. Try to get a new sample, measure the quality and performe your PCR.
Maybe you must change your conditions; you can taste with several annealing temperature.
It´s all friend.
I have designed a set of primer pairs for a particular gene. The first PCR result showed positive result for this gene but the band was weak. A second attempt on amplifying the gene with the same set of primer pairs did not generate any bands at all. Would like to know the reason why a positive result on the first attempt but a negative outcome after the second try of the PCR.