Slack in a system is often overcome in one direction only by the rotation of your cutter. This will present itself as a flat on one side only.
When doing test cuts, if possible, use a plotter pen or ball point pen chucked in your tool holder. ( remember not to power the router on).
Easier to try, You might try to also cut your test circle ( or square in two passes on opposite directions. This will mildly exagerate the slack because of cutter rotation and exibit itself on the horizontal parting line between the two cuts. Keep the overall size of your cuts down to less than caliper span. The error on a 1" cut will be the same as on a 16" cut if it mechanical.
You might also send your g-code to a willing volunteer with a known tight machine and re-assure yourself the code is good.
If none of this defines your problem, then you have control problems.
Best of luck,
Phill Pittman
digicarve@verizon.net www.masterwerkes.com