Walt - I have read all these suggestions and whilst I am not confused and they are all good suggestions, lets get down to the nitty gritty.
You have the machine in front of you - what is it doing wrong. Is it cutting too small or too large - and from that, does your axis appear to miss or gains steps. That is the first step.
Having said that, and I missed it first time round, plastic is very difficult to cut. As daft as it seems, the cutters must be very sharp, and the speeds must be properly controlled. (I only say this because my local plastic supplier who uses CNC said so).
I think I would try the cut manually first, say setting the cutter at exactly the right position with the spindle running, and then jogging the cutter up the cut, using a low percentage of the jog speed to do it with. Then measure the result and see if it is accurate.
If you CAN cut it accurately, then you can progress and see why the program won't do it.
JIm