When you are cutting plate, the quality of the material makes up about 75% to 80% of the application. You must have good material quality (free or rust, scale, paint marks), beam-mode, external optics aligned, low gas pressure (0.5 Bar or kg), and last but not least your lens must be in good conditions.
A36 is usually a lower quality steel in comparison to A572. This doesn't mean you can't cut it, but sometimes depending on the material composition, some may cut faster than others (taking away the other factors mentioned by Serguei and above). You may have to slow down.
The green surface that Serguei talks about sounds like a Cold Rolled Plate or Hot Rolled Pickled and Oiled to prevent rust. Try slowing down the feedrate by 15% and remember to also tune the power down to avoid overheating. Use an average of 2500-3000 Watts which should be more than plenty.
If you have the time, check the resonator for leaks, the power output using a power meter (if available), double check your external optics to make sure they are in good condition, and last but not least, double check your lens for heat stress using a pair of circular polarizing lenses.
Best of Luck