If you know a particular unit that you have rebuilt the carb on, I'd try buying a new carb for that one to see if it fixes things. If possible, get an adjustable carb replacement, so in the future you can adjust it. If the new carb fixes it, but the rebuild didn't, you've learned something - your rebuild isn't working, but a new carb is.
I don't know how you are rebuilding carbs, but there are a couple of things that might make a difference:
1) spray out the passages with carb cleaner -this can clean out holes you can't get to
2) Buy an ultrasonic carb cleaner and run the disassembled carb thru that several times, with a few drops of dishwashing liquid, or a degreaser on the water. You might be surprised what comes out
3) Seal the welch plugs with fingernali polish or indian head shellac before putting the carb back together
4) Check the height of the needle valve and adjust if not correct.
5) Make sure you put the diaphragms/gaskets back in the correct order. Laying them out carefully or having an IPL in front of you during re-assembly will help with this.
Also, check muffler screens and exhaust ports for carbon build-up, check for soft or blocked fuel lines and fuel filters, dirty air filters or spark plugs. Tygon is good replacement fuel line that isn't affected by ethanol.
Loose screws on the intake, or a leaking gasket can cause it to bog, too, but this shouldn't be happening on all of them at once.
Look for a gas station that sells non-ethanol gas at "puregas.org".
I would take one machine and methodically go through all these things until you discover the problem, then apply that knowledge to all of them.
Sometimes you can get a replacement carb on fleaBay or a small engine supplier far cheaper than at your local dealer. I've bought a brand new carb for a Stihl MS250 for less than $20 that way. You may find out the new carb is cheaper than the cost of a rebuild kit, plus your time.
Would still be helpful to go thru a machine one step at a time to help you figure out what is wrong.