Problems with PCR from cDNA samples

After retrotranscription I've purified my cDNA samples and quantified them having concentrations ranging from 50ng/ul to 150ug/ul. Controls which have a 50ng/ul have produced good quality amplifications but, the other samples didn't produce any pcr product even when they were diluted to the same concentration as the controls. I'm using primers of a constitutive gene and I don't get any result.
What could be the problem on these samples that do not amplify?