Asking a single, un-zoned HVAC system and a single thermostat to condition multiple levels to the same temperature is a tall order. It's an even taller order when one of the levels is a finished basement since basements typically have small heating and cooling loads, but there is a need for dehumidification.

Adjusting balancing dampers, if you have them, is likely to become a fulltime job, but it would be a great Monty Python skit.

Depending on your air handler and how your ductwork is configured, you may be able to retrofit a zone-control system with a thermostat on each level, thermostatically-controlled dampers, and a zone controller. That's major surgery and a big investment, but it's a proven strategy for conditioning multiple levels utilizing a single air handler.

Another, less-invasive alternative that will improve comfort is to install a thermostat compatible with multiple temperature sensors and install one on each level. You can configure the thermostat to control based on one sensor at one time of day and the others at another time of day or you can average them. It will improve comfort, but be prepared for the increase in your utility bill. Another option is to install dedicated systems for each level.

Most of the heat gained in attics is radiant heat. You're not going to "flush" that away with an attic fan. I'd leave the darn thing turned off. Air sealing and adding attic insulation are typically the most cost-effective ways to improve comfort and reduce heating and air conditioning costs. Many utility companies offer free or low-cost home energy audits. The tech performing the audit can help you make an informed decision about how much and what kind of insulation to add. If your utility company doesn't offer home energy audits, you can look for a HERS rater in your area.

HVAC is largely a function of design. How well the design is, the better the performance.

Heat rises. The bigger the structure, more heat. HVAC doesn't produce cool, it removes heat.

The more heat there is to remove, the more the HVAC must work to remove the heat.

Complications of design: 2 floors or more with only 1 HVAC system.

1. Thermostat location: on the lower level the thermostat reaches temp and shuts off. (it doesn't know what is going on up there on that 2nd floor. (Heat rises)

2. Close off vents to force air to upper floor. This is not going to be practical for all that long in my opinion. And doing so may cause your system more harm than good. (there will be those that claim success in these endeavors, but each system / climate that system resides in varies like the winds that blow and stop.)

3. Don't Move thermostat to top floor: now the 1st floor is too cold (or some other problem) Design problems are not solved by this kind of thinking. (re-design or live with what you have)

4. The addition of a zoned system. If you're gonna do it right, this method is laced in arguments and in some cases a serious amount of looney tune rhetoric. It does work, but requires skill. It is a form of Priceless Art. Requires room, should be elegant / not haphazard / Many will agree to disagree because alot in this space of HVAC zoned systems is to put this as eloquently as possible: Pure Junk. (realize this zoned system is a tool in the tool box, not all homes are a good fit for a HVAC zoned system. Is somebody going to sell you something to fix the problem or just sell you to sell you? Keep this in mind)

5. Zone your cooling areas individually with New Equipment. This elevates the cost provided of course you have alot of room, the budget etc. You can't re-use the current system you have now -- as that would likely add more insult to injury. Over sizing HVAC is never recommended. You will need two brand new systems to pull this off. All new duct work, separate dedicated electric circuits, gas circuits if your furnaces are natural gas / or dedicated electric service if they are heat pumps and seperate controls for each system.

6. (Temporary option) If your budget is such / area allows window units you could put in window unit on that upper level to help with the heat load up there. This would be considered more of a tempory solution until you decide how serious you are going to be about correcting this HVAC Design flaw.

I service the Katy, Texas area.

The list of possible reasons is long and impossible to nail down without on-site testing and inspection. If this problem is new that would narrow possibilities somewhat. Have you hired a pro to look at this? If you must have a recommendation to feel you got your money's worth, try putting a booster fan in top floor return air. If there is no return on top floor, you've found the main problem.

Could this be an attic fan issue? If it's not working properly, could the attic be overheating and affecting the top floor temperature? Is this potentially an attic insulation problem?

It is unlikely a hot attic would cause an 11 degree temperature difference.

HVAC - single zone. House is 3500 sq ft including basement

What are the areas of the first floor and second floor? Is the furnace or air handler located in the basement or first floor?

Is this a recent problem or this something that occurs each summer?

How is the air flow coming out of the second floor registers compared to the first floor?

It is possible you have a two zone system and one or more damper controllers are no longer operating. This type of problem is difficult to troubleshoot remotely. As others have suggested you may need to hire an HVAC company to determine a viable solution.