The best way to test it would be to make a "free vend" switch.  If it vends with the switch then it is your changer.  There is a sticky at the top of the coin mechanism area that will show you how to build one.

It will work on a Select-O-Matic.  You make the free-vend switch and when you press the button it will (should) vend what ever the dial is on.  Here is the info on it.

Ok, good , you've checked the vending motor & it's cycle switch ( which parks the motor/rack assy at it's "home" postion , also check the empty switch & the empty lever/bar that triggers it, on the Westinghouse Wc 60/78/102 -MD series (Select-a-matics) it is the only thing between the coinmech & the vending relay tha can prevend a vend cycle, check your empty switch carefully, if it's not correctly set on the "empty" bar (triggered by the empty paddles in each column) it will also prevent a vend. sometimes the switches trigger-lever will get bent the wrong way & the machine will think it's on an empty column, ( is the "make another selection" light on?/ or does it trigger ( the empty light) when the knob is rotatated form one selection to another) ...just a thought.