how could i know if the compressor is not functioning properly, when i charge with ferion the the a/c cools but in the third week the cooling level decreases until heat comes back and ive compressed the a/c system with air and checked if there was any leakage and checked the air pressure after 2 hours and there was no leakage so can any one tell me what could be the problem??

Sounds to me like you may have a very small leak somewhere that works initially but de pressurises over time. If you get cooling then I guess the compressor is doing its job.

there isnt any leak but something elses and i need ti know what is it

Does the pressure in the system decrease and require topping up after several weeks? If so you have a leak.

Otherwise I would look into a compressor or inspect the hoses make sure none are damaged or perished.