Hello, the odds are in the 100% range that the condenser coil (radiator) at the unit on the roof is partially clogged with dirt deep between the fins where it cannot be seen. Cleaning the condenser is a must. If the unit is in workable condition otherwise that will increase the cooling capacity by 30% or more.
Click here for a video.
You can buy the condenser coil cleaning kit at most larger hardware or home fix up type stores.
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