You may have gotten caught up in one of the mysteries of the modern world. While all flappers may appear to the be same in reality such is not the case.
If you will examine the underside of a flapper valve you will see an inverted cup with a hole in the bottom. When the flapper is at rest in the closed postion air enters the hole in the flapper cup from below. When the flapper is lifted that trapped air forms a bubble that holds the flapper in the vertical position until the water level in the bowl is below the opening of the flapper cup. Once the water level is below the flapper the air bubble can no longer support the flapper and the physical weight of the flapper causes it to drop down to the closed position.
Up until the mid 1980's all water closets consumed an average of 4 to 4.5 gallons per flush and with the growing concerns for water conservation the industry begin finding ways to conserve the amount of water consumed by the flushing action.
In the early days of water conservation toilets they simply reduced the overall size of the tanks while keeping all other physical dimensions of the toilet basically the same. In this manner they got the average down to 3.5gpf.
In order for a toilet to flush properly the water stored in the tank needs to discharge into the bowl quickly, which then raises the water level in the bowl above the top of the trap and the water spills out the back of the trap in a large slug that begins a syphon action. This syphon action continues until almost all of the water in the bowl is sucked up through the trap, which finally permits some air to enter the discharge port in the bottom of the bowl, rise up in the trap and break the syphon action. When this occurs a small amount of the water in the trap then backflows back into the bowl, but the level is still too low to insure a good gas tight seal in the trap, so the fill valve has a small plastic by pass line called the "Trap Primer" that allows a portion of the incoming fill water to flow down the standpipe and into the bowl to refill the trap.
The government then gave the industry a target of finding ways to build a toilet that would only consume 1.6 gal/flush. In order to meet the new standard they again reduced the size of the tanks, however it was discovered that the common flapper was leaving far too much water in the bottom of the tank, so the flappers were then redesigned with a smaller hole in the inverted cup. This meant that the bubble would remain in the flapper until the water level in the tank was almost empty thus insuring all the water in the tank was properly discharged to the bowl.
While reducing the tank size and redesigning the flappers had resolved the problem of how to start a flush with 1.6 gallon of water many of the early "Water saver" toilets had problems with incomplete flushes, which often required a second or third flush to completely clear any solids in the bowl.
It was then realized that while they had resolved the problems with how to discharge the water to the bowl they had not considered the size of the traps. Realizing that a water closet must be capable of handling solid waste reducing the diameter of the trap was out of the question, however they could easily reduce the vertical height of the trap profile which would in turn reduce the amount of water required to start the syphon action.
The problem now is that while the toilet manufacturers have done an excellant job of redesigning the system, they have not bothered to properly inform the parts retailers or the general public as to the changes in the design. As a consequence most people determine they need a new flapper and simply go to the neighborhood hardware and grab the same old reliable standard flapper that has been in use since their grandfathers time, not knowing that the new high tech toilets require a flapper matched to the Gallon Per Flush rating of thier toilets, and once the wrong flapper is installed it continues to perpetuate the myth that the new water saver toilets do not work properly.
There is now another condition that causes a lot of confusion to the end user. In order to acheive maximum water conservation many water closets now have a two stage flushing system. If the flush handle is quickly depressed and released the flapper only releases .9 gal of water into the bowl. This is normally sufficient to handle liquid waste (urine) but if a complete flush is required to discharge solid waste you must depress the flush handle and hold it down for about 1 second before releasing it.
In fact, according to a recent Consumer Reports study one the most reliable and hottest selling toilets on the market today is also one of the cheapest in the industry. That being the American Standard "Cadet II" which has an average retail price of $60 complete.
Fluidmaster has now come out with a universal replacement flapper that has the rubber ball attached to a stiff plastic framework. The hole on the underside of the flapper ball is slightly off centered and the flapper can be adjusted to the proper GPF rate by simply rotating the rubber ball on the mount. While the actual cost of this type of flapper is about twice that of a conventional flapper the price is still relatively cheap at about $4 and many service plumbers and handymen are relying upon it as a universal part rather than needing to carry three different types of flappers on the service truck.