Opto not recognizing thin sheet

Has anyone had problems with Opto's not responding to thin sheet? I hooked up my opto's and passed my thin sheet through all of them. Two of the opto's will turn off the led when the thin sheet (soda can) passes through the uprights. The third one will not turn off the light when the thin sheet is passed through. I hooked up the opto to my mendel and jogged the axis to where the thin sheet should have stopped the axis from moving but i was able to job the axis further until i bent the thin sheet.
I am going to double check my connections again tonight when I get home. Have any of you seen this before? I am thinking that the upright sensor thing is busted at least. I sent a pm to Kimberly to ask her about this an to possible get/buy a new one.

check your RJ45 cables, then try putting it in max instead of min or vice versa, otherwise if the sheet needs to be slightly thicker maybe put some electrical tape around that bit of the sheet to make it wider that's all i can think of

The sheet should also be non-reflective to IR otherwise stray light can reflect in and cause false triggering.

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I will try the min to max test, thats a good one. I was using the same piece of can to test all the opto's and the other ones recognized it, I even used a piece of thick cardboard and that worked all all others except the one in question.