Agencies & Money

What's with some of these agencies out there. I worked for one for 3 weeks and every week there's been a cock up with my pay, somehow they've always managed to short change me and pay me for less hours than I've worked. I keep a spreadsheet of all my hours and sent them copies a number of times and promises are made to correct it but nothing happens. Frankly I'm getting sick and tired of this and unless it's all corrected today I won't work for them again, just can't trust the payroll dept to get it right.