Non payment, 1099 work

I am a handyman in Nh. I do apartment turnovers. For the past two years I have been submitting my invoices and when I get paid the total due is not the same as what I submitted. They are deducting mony from my invoice. Now I am also not getting paid on time. Invoices are due in on Monday and to be paid on Friday. I turn in my invoices on Monday and am not getting paid till two or three weeks later. When I asked them about it I was told that he forgot to give it to the accountant. When I asked about why they are paying me less then what I submitted, they are telling me that they have a guy that does work for them that does the work faster. Now this guy had a crew and I am a one man show. They know this and still pay me less them what a put on my invoice. Do they have the right to make changes on my invoice without my permission ? And do they have the right to with hold payment, when I turnedin the paperwork on time?
Thank you