wrong plot amplification in one-step RT- PCR - one step real time RT PCR (Jul/10/2011 )

Hi everyboody

I am working with double strand RNA isolated from feces using a kit from qiagen. I have checked the concentration and purity by nanodrop (aprox 85 ng/ul).
For detecting virus presence I used different probes and primer concentrations together with quantitec-probe RT PCR (qiagen) and I add 5 ul of RNA sample.
The curve showed in each analysis/test did not have a sigmoideal shape (linear view)but looks like 1/4 of a circle, it seem they never get the plateau stage.

My NTC don't show any curve . what can I do? How can I improve it? I will appreciate any suggestion.
thanks in advance.

Do you have a screenshot?