Normally that is a result of a flush handle to flapper chain that is a bit too long. The handle lever drops before the flapper does and a bit of the chain gets sucked into the flush valve, which in turn prevents the flapper from properly seating when it drops.
If you will lift the lid and examine the chain, it should be fairly straight down from the handle arm to the flapper. If it appears to be really loose you can open the safety pin type clip on the handle and take up a couple links of the chain to adjust it.
The proper lenght of the chain should be the lenght from the flapper at rest in the seated position to the handle in the at rest position + about two links of slack.
