nicktheplumber
Some flappers have an air chamber that keeps them afloat for a certain amount of time. These chambers can leak or get clogged, which screws up the timing for them to seat in the tank outflow drain (the one that sends the water in the tank into the bowl). Try adjusting the chain linkage from the flush lever to the flapper, as RP suggests. If that doesn’t fix it, replace the flapper or ball valve.
NtP
