I know it's really hot (105 today), but my ac is not keeping up with what I set it to during the day. Yesterday I set it to 78 when I left for the day and when I came home at 6pm it was 81 in the house. I set it to 75 and it did't reach that temperature until 11pm. This isn't normal even though it's extremely hot right?

Have you changed your filter latly? Sometimes just a clogged filter can inhibite the units performance.
If it was further than three degrees there might be a different issue. Try changing filter first. Alsoo not any changes, sounds, or issues.

I am having same problem. I am in Austin, TX. Outside temperature has been 100-1005 for the last month. By 5 PM, inside temp upstairs could not get past 80F, my setting is 75F. By late night, it is able to reach 75.

Downstairs is also set at 75F and works just fine.

I called the same company that installed the A/C, they sent out a technician, after all kind of checking, he concludes there is nothing wrong with my system. No leak, freon is virtually full, I was charged $86. I told him how come it worked fine previous years. He said temp was not that hot previous years.

The house is 4 years old (new when I bought it) ? WHat can I do, all filters are regularly changed every 3 months or less. Even after I changed all new filters, no improvement seen at all.

There is an air exchange in the system which when activated will suck air in from outside, I went as far as disconnect it to make sure that will not happen. No improvement either.

What can I do ?

Is your tstat upstairs or down stairs?

One tstat upstairs, another one downstairs. There are 2 separate A/C units.