Massachusetts law requires all employees must be paid within six days of the end of the pay period during which wages were earned. For example, if a pay period ends on a Friday, employees must receive all wages earned during that pay period, including overtime, by the following Thursday.

As far as the law is concerned though you are paid the moment the check is put in a mailbox to be delivered by the postal service.

I noticed you wrote, "...the office in Providence, RI where the checks usually come from...". Do the checks not always come from the same office? It also wouldn't surprise me if a RI HR & Payroll department doesn't know the law in MA. Not that that is an excuse but you might want to point it out to them.

As far as not being paid for 6 hours you worked. That could simply be a clerical error. You mentioned you supervisor dropping off time sheets. Did you talk to your supervisor about the missing hours? He may have to sign off on them.