There could be many reasons for this, the things to look for are; do you have a return (a duct with a non-adjustable register over it), if you don't the air will not be recirculated and cooling will be less effective. A simple fix is leaving the door open if there's a return in the hallway. If you do have a return make sure it's not blocked, the same goes for the supply, make sure the register is open all the way and not obstructed. It may just be your room is warmer because it's on a side of the house with more exposure to the sun, the windows, distance from the air handler/evaporator etc. Most systems have dampers on the supplies that can be adjusted to increase flow to different areas, these can be balanced (adjusted) to equalize the temperature in different areas of the home. Booster fans can also be added to increase flow to rooms farther from the air handler/evaporator. But since you don't own the house I don't know how much you would be allowed to do, or want to pay for. A simple solution might be buying a small window unit for your room, they're cheap to buy and run.
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