Sigma Lens For Canon Slr Camera

John asks…why does my sigma 75 300 mm lens only work about 1/4 of the time on my canon rebel ti 35mm?I am just staring out in photography and i bought a 35mm canon rebel ti,it's a great slr camera and i wanted to get use to a slr before i went digital.the camera works fine with everyother lens i have,i bought this 75 300 sigma lens off ebay and it only works about 1/4 ov the time,when i go to shoot a pic it just clicks and no picture is taken.i was wondering if i am doing something wrong with

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