I receive a salary based on a 40hr work week. When I work more than 40 hrs in one week I get paid 1x hourly rate + $6 for each hour over 40 hrs. This past week I only worked 28 hrs because not enough shifts for me to work. They told me don't worry you will still get paid for 40hrs but you will have to make up the 12 hrs in the future. So if I work 52 hours next week don't they have to pay me the 12 hrs at 1x salary +$6 or can they only pay me for 40 hrs and say I owe them 12 hrs. Doesn't seem fair??

Sounds like you are being paid under the Fluctuating Workweek method. I would note, though, the statement here regarding the "average hourly rate" for any given workweek cannot fall below minimum wage.
http://www.dol.gov/dol/allcfr/ESA/Title_29/Part_778/29CFR778.114.htm

It is not a violation of the regulation to require you to "make up" the time when you work less than 40 hours in a workweek. However, each workweek stands alone. So if you work 30 hours this week, you get your regular salary. If you work 50 hours next week, you get your regular salary plus 10 hours overtime at the "regular rate of pay" (salary / 50 hrs). The regulation does not allow the employer to offset the 10 hours short in one week against the extra 10 hours worked in a subsequent week with no additional pay.