2001 Chevy Silverado A/C takes a small amount of charge, bu

I have added R1234a with oil mix to my A/C system and some charge enters the system to give cool air in the cabin. The pressures drop to the 30 range when the compressor comes on. Days later the amount of cool the air gives has dropped. Do think my system is leaking R134a or was the initial cooling effect due to the fluid entering the system?