Replace AC or something else?

[Max Biggar]

Hello BP, Looking for some advice here. I just closed on my first rental property SFR in Jacksonville FL about a month ago and am now having an AC issue. The tenant called and said the house isn't cooling. I had an AC company out right away to inspect and they said the unit seemed to be operating properly but was a little low on refrigerant. I had them top it off and paid the $300 bill (emergency call on a Sunday).

 Just got another call from the tenant (1 week later) and they are having the same issue. I had the AC company go back and they said everything seems to be working properly and there's nothing they can do. It cools the house down to 75F at night but during the day struggles to go below 84F. 

It is central air and the unit is >20 years old but still running. I'm not sure if I need to replace the AC system with a new one since it's 20 years old or if I will have the same problem. Or is there an inspector or someone that can check the duct work or air leakage? Does anyone have a good contractor that does that sort of thing? 

Thanks for any advice you can provide!

[Max Biggar]

Thanks everyone for your helpful input!! I had another company out today and they were able to find one of the ducts was half open at a connection point sending cool air into the attic. The ducts are old and were in bad condition likely trampled by contractors over the years and just repaired with duct tape. I am getting a quote to replace all the duct work. There was also no insulation in the attic, so I am getting a quote for that as well which should also help. 

The AC unit appears to be working well and is a 3 ton unit for a 1400sqft house which should be plenty big enough so that at least can stay.