What is going on with my central air?
List of possible causes and fixes:


Insufficient airflow. 

You have identified this issue but not the cause.
1) Take all air filters out of the system. Give the system 30-40 minutes and see if this fixes the issue.
2) Ensure all vents are fully open.
3) Verify and/or adjust the cooling blower's air speed. WARNING: Turn the power off to the HVAC before doing that. The only way to verify or adjust the speed is to get the manual and find the table that lists the dip switch settings. Somewhere on the circuit board under the cover of your air handler there will be a tiny block of dip switches that control various things like the blower's speed for cooling and heating. A lot of shady service men will set this too low and continue to charge you for service calls. Then there is the group that doesn't even know it exists. ugh.. If you do change it set it to the middle-high setting. Usually there is low, medium-low, medium-high, and high.
Leaking freon. 

You might have a tiny pin hole in the copper some where. Your service guys indicated this was ok. If you trust them then this is not the issue. 
Copper tube is not covered with insulation.

Go to the box store now and get some black "pipe insulation" where the copper tubes are located and then cut it to cover the tube. This is a four minute job and will at least prevent the tube from freezing; which can damage your system by the way.
You condenser coils are ineffective because they are full of debris like grass and dirt.

The condenser is the big unit outside and if its clogged with dirt it will not allow the freon to release its captured heat from your house. You can fix this yourself by either spraying the coils with a water hose or by renting a pressure washer that allows you to add soap to the water slowly. This happened to me last year because of grass clippings and dirt build-up and this can happen to anyone regardless of the age of the HVAC system.
1) Make sure you spray from the inside out. You must take the cover that holds the fan off and the shroud/cage off too. Why? Because if you spray from the outside towards the inside your just pushing the debris further into the coils possible lodging it in there forever.
2) If you go for the pressure washer DO NOT get too close with the nozzle. If you put too much pressure on the coils they will fold and you'll ruin your unit. Pressure washers come with various sized nozzles and you should use the biggest nozzle first and if it isn't working then move your way down to the next smallest nozzle. Bigger nozzle = less force. Either way you need to get some soap mixing with the water at a good pressure in order to clean the coils. Its best if you take the shroud off so you can get to every square inch of the coils.
Your compressor or fan is not turning on.

1) Check the wires that run to your unit. A lot of times there is no cable management for these wires and they are just dangling in the elements as they run from your house's foundation to the unit. They can easily be cut by a weed-wacker. Re-connect/solder if this is the case. Make sure you use some heat shrink tubing to protect the connection from the elements.
2) Your relay switch is broken. Lots of times small critters crawl into the relay and get electrocuted when the relay kicks in causing a small explosion that's powerful enough to crack and melt the plastic relay and the metalic parts just enough that it stops functioning. The easiest thing to do there is get a replacement. Service guy should have already checked this but who knows if yours did..
3) Your compressor has failed. Seek professional advice.
4) The fan has failed. Seek professional advice or look at potential wiring issue.