Poor management, poor payroll, poor hygiene, poor culture/environment.
Never get any help from managers or HR even when grievances are made. You WILL get underpaid and you will have to chase this up in your own time for weeks before getting any response, only to give up and let the thieving company keep the money you have earned.
Would get generic responces along the lines of "we try to get it right all of the time, there will be times we fall short."
...Well one or two mistakes can be forgiven, however this company will never learn form its mistakes as they refuse to listen in the first place.
If you want to help your employees then do it instead of brushing them off. Id happy to write to you (again) if you leave me contact details of someone who is actually going to do something.