I’ve noticed that the AC won’t get cold if it’s not over about 90 deg outside. This is even with it set to “Lo”. On a hot day, it gets pretty cold after a few minutes of driving. I know there’s an ambient temp sensor on the dash for the climate control, but shouldn’t the “Lo” setting force the AC to run maximally no matter what the ambient temperature is?

 

How long has this been happening? Is this a new thing or been happening? New car or had it a long time?
Front and rear AC? What year, make and model?
Start the car.
Turn on AC on full blast
Open hood
Do you hear the compressor kick on? If not have an assistant turn off/on the AC from the Cab you should hear it click and see the compressor "wheel" start to turn when it's on.
When the compressor kicks on, does the electric fan come on? It should. (there are two fans on a disco, one in front of the engine and one electric fan that is for your AC condenser (in front of the radiator)

A few years ago I made the plunge to do AC work, If you do this, you need some tools - AC gauges:

You either invest a bit and can fix AC at home (it's surprisingly easy) - or you take it to a shop.

Put the gauges on, and see what the low and high side pressures are.

 

I'd go to AutoZone or O'Reillys and get a set of manifold gauges for "free" with their loaner program, check your refrigerants and compare them with a R134 refrigerant chart. That's the most definitive check you can do. When refrigerant levels get off, I've known my A/C clutch to do funny things. Plenty of videos on YouTube on how to connect the gauges properly, ericthecarguy and chrisfix have good ones.

 

My guess is that electric condenser fan is not kicking on and putting any air flow across the condenser until the Ambient Air Temperature is hot enough that it kicks on through the engine temperature and not compressor demand. The system is designed to turn that fan on in one of two situations, one where the air conditioning comes on and the other where the engine heats up till about 200°. If that fan does not come on you won't get very much cooling capacity because there's no air flow across the condenser

 

yes this is correct, the low shedding should turn on the compressor until the low pressure switch turns it off and also should force the temperature blend doors to the full cold position