Check the insulation in the attic. You want at least an R30 insulation value, and preferably R49. While you are up there check on how the attic is ventilated. You may need to add more vents to improve air flow.

The garage ceiling must be well insulated. If it is not all the hot air in the garage will rise up into your bedroom. The garage and attic create a sandwich of hot air. This is why the master bedroom is the hottest room in the house.

I was planning to check the insulation in the ceiling this weekend. I'll make sure to check the attic as well.thanks!

the issue is temperature gain from the extreme attic temps
that surround these rooms.
we generally use spray foam on roofline..high cost
but fast...& pay lots of attention to details of foam install.
must be air tight to outside as you are creating an unvented
semi conditioned attic..depth & type of foam must reach
recommended R-values of your area.

other than that...you can add a reflective foam board
at all walls shared with attic space (over insulation..
caulked to air seal, taped seams & attached with
button cap nails). this will add insulation value to the walls,
air seal & reflective coating on foam sheathing will reflect
heat back into attic space.

search southface's website for air sealing pdf's.
this wall shared with attic space is called a kneewall.
which also applies to the difference in ceiling heights
of interior rooms in attic space.

as your central unit runs...it pulls attic air/temps into
these walls open to extreme attic conditions.
stopping the heat gain & air infiltration is the
best investment.

depending upon how well your existing ductwork is
sealed...and average home has minimum of 30%
duct loss...mastic seal of ducts is also a quick
return on investment & improvement in comfort.

best of luck.

The room over the garage cooling problem is a very common one. Agree with comments regarding better insulation to decrease heat gains as the best solution for the long run.

If your looking for the cheapest way to get better cooling consider a window air conditioner albeit you'll deal with more noise.

A second idea might be a mini split AC or heat pump unit for that room. Mini splits are very quiet and very energy efficient.

Third idea would be creating another zone just for the master bed room (assuming you don't already have that).
Assuming your present AC system wasn't already running almost 100% of the time during the hottest days zoning would help. This might lead to the highest electrical bills though.

you either fix the problem
or purchase more a/c to cool the area.
the first is a one time cost with lifetime benefits
the latter is more upfront costs & higher costs
every month to operate.

search the site for F.R.O.G. (family room over garage)
threads...homeowners always find these areas
difficult to condition.

best of luck.