A slight under extrusion could be a likely cause. If the part is 30mm tall then that should be 150 layers at 0.2mm per layer. Imagine now how little of a decrease in layer height it takes when spreading 0.2mm over 150 layers.
I would agree with this theory if we're talking X or Y dimensions.....but not for the z axis - the layer height is a function of the physical height of the nozzle above the bed. Extreme under-extrusion may result in layers not binding together...not an under-height part!
Reading the reported issue leaves me with a couple of permutations....
If the printing IS DEFINITELY not printing the top one or two layers....then you need to check you have not got the Stop printing at Height tickbox on the Advanced tab ticked. If not checked then you need to take a look at the suggestions here: https://www.simplify3d.com/support/prin ... mid-print/
If you're getting all the layers printed and the model falls short in height, then you most probably have an issue with the calibration of the Z axis stepper motor. You've not said what your printer is. If its a factory calibrated printer, I'm surprised you're having issues. If you have a DIY build or custom built printer then you need to recalibrate your Z stepper motor...and you'll need to follow the calibration instructions from your printer supplier. Here's a couple of search results on the forum that may give some background info:
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