Hi - my flapper doesn't stay open long enough for the water to escape to flush. The flapper immediately reseats itself blocking the escape of water. I tried changing the flapper but same problem.

See the image below this is how you install a flapper and adjust the chain to 1/2" slack, good luck.

John

If the flapper does not go up high enough this is the most common cause, is the chain too long ?

Sounds like your flapper linkage needs adjusting.
With the flapper seated the linkage wants to have 1/4" of play. Too much play and the flapper doesn't get pulled back enough giving you a short flush. Too tight and it lets water seep past the flapper. I adjust the chain by the link and if it needs fine tuning, I bend the flush lever rod a bit until I have the desired play in the linkage. One more thing about a flapper. you will see where the old flapper hooks onto the base of the white overflow tube. On a older type with no hooks the flapper has a ring that slips down over the overflow tube to the seat,(see image). If your tank has hooks, take a sharp knife or scissors and cut the neoprene ring off on the marks provided and hook the flapper on the hooks. Leaving the ring on will interfere with the flush. The water level in your tank should be 3/8 to 1/2" below the top of the overflow tube when the tank's filled. And the little tube wants to run from the ballcock and connect to the white overflow tube.. In addition to the above if the flapper has a ring it must be cut off, (see image), Good luck, Tom