A/C Troubleshooting Help

[arriflex]

Hey gang, last month we noticed our electric bill jump a bit, I thought it was just the summer heat coming on. We live in a rented 3 BR top floor apartment in Florida (Tampa area). We have been out of town for a week (until Sunday) and left our thermostat on 78 while we were gone (we have pets). The last few days our A/C unit has been running almost non stop, unable to get the temp below 77 degrees. It runs all night until about 3 or 4 am to get the temp to 75. During the day it can't crack 77.
Maintenance has been by twice, they replaced the filter, checked pressure, and checked separation (18 degrees) and told us everything is fine, the unit is working as it should.
We spoke to a friend in an identical unit, she said she keeps her thermostat at 72 and her A/C gets there and kicks off during the day just fine.
I suspect some sort of duct or circulation issue, but am curious to hear your thoughts. We dont have much experience in this, and it is frustrating- the unit runs all the time and can barely keep ahead of things. Last night the temperature actually climbed after dark- up to 78 at 9:30.
Thanks for your insight,
K

[HayZee518]

a few things come to mind. this I take it is a central a/c unit. check the heat exchanger coils of the condenser unit. they should be clean - no leaves etc. if its a water cooled condenser check the incoming water temp - should be 15 degrees cooler than the surrounding air [ambient] if there is a manifold that directs the freon to the evaporator then have your tech check the thermal expansion valve. the bulb of the TEX should be mounted at the bottom of the hi side line.