How to provide cam follower mechanism for flat surface

Hello, I am using CREO 7 and currently doing the cam follower mechanism for a cam rod and tappet and was following the tutorial provided PTCU. The tappet is having a flat surface. The mechanism works well for curved follower but not for the flat surface. I gave the necessary parameters that are required to define the mechanism but I am not getting the usual motion that is expected from this mechanism. The tappet sticks to the cam surface and rotates along with the cam rotation, which I don't want (I need the reciprocating motion). What should I do? Are there other parameters need to get the required cam motion?