Well there can be 2 problems.
1 - Clogged Condensor or condensor fan not throwing enough air at idle.
2 - The compressor is weak and no longer working at low rpm.
A/C is ac it should work at slow,fasht, in heat, in day, In night, In summers, In winters , blah blah
Baki ac agar garmi main kaam nahi karta tu phir dil ko bahlana acha hai ka 1000cc ka kam nahi karta