New construction requirements for low flow toilet?

What are the venting requirements for the American Standard Cadet #3600?

I have recently moved into a new home and the toilets are a little sluggish and sometimes do not flush. It helps if you hold the lever to the count of three. I spoke with the plumber about this and he says...."sometimes they just do that....even if you spend $500.00".
I've already exchanged one of the units during the installation process, because it did not flush at all. A friend suggested that it sounded like a venting problem.
I did not see ANY venting in the attic except for the sinks and the water heater. I do not want to over-react if the new low flow systems do not require it. Could he (the plumber) have possibly vented it into the sink vents?? What should I be looking for?
Please advise ASAP.

KM

Toilets should be vented with at least 2" pipe.

The vents can be connected at 42" above the floor, so you may only see one vent per bathroom set.

If you have to hold the handle down, you may want to check the flapper and float for it. Most will let about 1.6 gallons from the tank, leaving quite a bit of water still there. That would be normal.

venting

Your friend is not a plumber. A toilet without a vent, assuming everything else is done correctly will flush better than one with a vent. The vent is to prevent it from "overflushing". Either the toilets are working the way they are supposed to, or you have a different problem, possibly in the toilet itself.