How to minimize delta ct value when everything else worked well in qRT PCR - (Dec/05/2017 )

Hello, greetings
My problem is that my ct value is very good and as per expectation for myto control). But as soon i consider normalisation with reference gene,the values of my delta ct, reverses the result (of absolut test groups (compared e expression of my test group). Also,the delta ct values are high of range 15 to 17. That is i got my ct values of test and control groups in range 31 to 33 while of reference gene it is of range 17 to 18. How to minimize this large gap (delta ct) between test and reference gene.
Thanks in advance

You have a rare amplicon? It looks too low for a control group.  If it is rare then probably the best approach is do a pre-amp (there are several kits for that). If your amplicon isn't rare then your concentration is too low.  Also look for another reference gene that have less copies, it should amplify somewhat similar to the controls.