Air conditioner problems

Hello,

It sounds like you might be low on Freon and or clogged condenser.
You should first check the Freon level. If it is low it needs to be recharged.
You get cool air first because the engine is cool and the under hood temperature is cool.
As the engine warms up the AC fails to keep up either for being low on Freon or the condenser is faulty.

Here is couple of helpful article of how to recharge your AC system:

https://www.2carpros.com/articles/air-conditioner-how-to-add-freon
https://www.2carpros.com/articles/re-charge-an-air-conditioner-system

https://youtu.be/uZrQCGwXfek

If the Freon level is correct then second item could be a clogged or damaged condenser.
If the condenser is faulty then you won't have proper amount of air flowing thru it to make the air cold out of the vent.

Please let me know if you have any questions.

Thank you.