Flag the bus early (but not too early)

Recently, there is this Flag The Bus Early Campaign initiated by SBS. But I found out today that you can't do that too early.

I put my hand out as usual (for at least 3 seconds) to flag down the bus before putting my hand down. The bus was some 2 bus length distance away when I did this. As the bus came toward the bus stop with no intention of stopping, I frantically tried to flag again. However, the bus just breezed through... Luckily, a second bus directly behind did call on the bus stop, and I was able to take this bus to my destination (though with a longer time...).

"Perhaps my hand should stay horizontal until the bus came to a stop before me..."

"Perhaps it was too dark that the bus captain cannot make out that I was flagging for his bus..."

"Perhaps the bus captain thought that I was flagging to the bus behind his..."

Whatever it is, I guess you would have to flag the bus at the right timing...