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HRF and Analysis Question

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Hi all, 

I've been watching these videos and doing the demos, which have been incredibly helpful--thank you! I'm having trouble understanding my data...I'm not sure if there is just "nothing" results-worthy or if I'm just not pre-processing adequately. My experiment is two blocks (two conditions). A stimulus appears on the screen and the participant responds within 30 seconds. If they don't respond by 30 secs, the trial ends and moves to the next. Most participants average about 10-15 seconds per trial. Between each trial is an ISI that is jittered for 2-12 seconds.

I have a screenshot of some of my parameters, attached. In (A) is the raw OD data. (B) Shows the first 500 sec since the stim marks are so frequent in A. (C) Shows the HRF for the condition using the processing stream in (D). And (E) shows the plot probe. It's clear from the plot probe that there isn't much. I have looked at scalp detection parameters and get good values, so I don't think the quality is poor. I am getting similar results for numerous subjects...and I'm mostly finding that the oxy goes way down and the deoxy goes way up. When I average the HRF at the group level, I'm left with pretty much nothing, as the subjects seem to cancel each other out. I have tried playing around with different motion parameters (I went through the motion correction videos several times). I also tried playing around with some of the GLM parameters, like changing the basis function. I find it hard to believe that my stimuli generate such little change in brain activity, so I'm a little stuck/lost (although I could of course be wrong). Any assistance, ideas, feedback would be greatly appreciated. I just want to make sure that this isn't me doing something wrong or neglecting something important rather than this being an authentic physiological response. 

Thank you!