maybe you forget to collect the fluorescence signal when the extension step. In additon,  the SYBR Green may not work.

Thanks for your reply Yunjing, but I process other samples, the positive controls, and they have signal as expected.

Do not waist your time. Try the Taqman system, it is highly reliable.

Best regards

AU-a

Thank you Carin. All of those possibilities have been covered: the same primers in control and experimental samples; one product is seen in the melting curve of positive controls, the same reaction mixture including primers is used for control and experimental samples.  I will try changing positions in the thermocycler to discard this variable.

Thanks Alfredo. I will try with Taqman system. It may be a matter of sensitivity. I expect a low level of expression in the experimental samples.

Regards