[mattdavis11]

Leaks are the main reason an a/c system will begin to lose effectiveness. The most common being the schraeder valves, both high and low. Most are the same as a bicycle, and if you remember as a kid, you had to air up the tires every spring. The sealing/design of the valve core has gotten better though. However, owners often have them checked, and every time they do, the valve core wears out. If it works fine, never have it checked by the drop out at the lube place. They could introduce some contaminants into the system as well. It's just a bad idea.

Another reason the system becomes less efficient would be debris on the outside of the condenser. They get pretty filthy as they are the first thing to catch what's in the air or flies up from the roadway. To dissipate heat/transfer, you need good air flow through the condenser.

Fans are next, sometimes the fail, sometimes just one fails, but either way this will lead to high pressures in a situation where the car is moving at slow speeds.

On your Camry, you may just lose the a/c altogether a day after it worked perfectly. It's a common problem. The coil dies and the compressor will no longer engage. Depending on what I find on the clutch end of the business, I might just replace the coil, sometimes I replace the clutch assembly (includes the coil), sometimes the whole compressor. The latter is the only that requires a discharge of the system.