B4 its been working fine, sort of. But this time it won't bevel this particular part. It basically adds something like a loop cut to the face.
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3$\begingroup$ Try converting all those ngons to quads before bevel. $\endgroup$ – Cebbi Jan 12 '17 at 10:29
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$\begingroup$ if nothing works, try to share at least a portion of your file (blend-exchange.giantcowfilms.com/)so others can inspect and perhaps resolve. $\endgroup$ – m.ardito Jan 12 '17 at 11:13
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I've had this issue before, I'm not sure if this will work for you but ctrl+a -> apply scale works for me
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Probably your problem is that the top part of the mesh is an ngon that bends around the corner connected to the edge you are trying to bevel.
I tried with a similar geometry: 
After dividing the top surface into two quadrilaterals, beveling works fine:
I hope this helps.
