No contrast differences in spm parametric modulation analysis

Hi all,
I have done a parametric modulation analysis in SPM. I have two conditions (cond1 and cond2) and two parametric modulators (pmod1 and pmod2). I have obtained significant result of ‘cond1_pmod1’
and ‘cond2_pmod1’. And I aimed to get the contrast of ‘cond1_pmod1 - cond2_pmod1’, but there was no cluster survived. Does anyone have a better way to get results that make a significant difference?