Room easy to warm with heater. But difficult to cool with A/C. Is this not insulation related?
Meet solar load

When the sun beats on your house, it warms it up - winter or summer.  (more in the summer).  There's 1000 watts of solar heat per square meter that's square-on with the sun, and only a limited ability of your roof to shed that heat - eventually some of it pushes through the insulation.  

This is a lot of heat, and you get the brunt of it being on the top floor.  

Needless to say, this is your friend in the winter and your enemy in the summer.  And it's a big enemy in the summer.

Part of the value of solar panels is it essentially puts parts of your roof in shade, reducing/eliminating that solar load (depending on how well air circulates around the panels). Having the panels racked a foot or two off your roof helps with this. 

As a renter, you have basically zero options to deal with this, unless you can sway the landlord somehow.