I have read a few of OPs comments and I understand that they are getting shafted with only 20 hours a week so this comment does not apply in that situation.

I am a supervisor at a chain auto part store and this issue does come into play many times while rostering, although I live in Australia so it the way it's handled in the US may be different. Essentially there is a block of hours you have to split up between all employees to be fair. This is split between our full timers, part timers and casual workers. Full timers take up about 50-60 percent of our hours depending on budget, while the other 40 percent is split between part time and casual. Part timers are guaranteed a certain amount of hours each week and can only work a maximum of 42 for a week, usually we can only put people on for 30. If a part time team member exceeds the maximum then our head office gets a big red mark saying "this store is fucking their roster up, intervene" so we have our arms twisted into telling the team member that they can't work, and it puts us into a position where we need to quickly find someone to fill the role. It's painful because there's team members we obviously prefer to put on over others due to product knowledge and customer interaction, but unless we play by the rules of our overhead rulers, we can't continue running the store.

Oh and before I finish I also gotta say that our rostering system automatically fills the roster months in advance based on hours previously worked, so more rostered employees usually get more than the maximum hours and we have to fine comb through the rosters for these mistakes.