when a minecart is on a unpowered rail and it becomes powered the minecart doesn't go

I am a map creator, and today I ran into a bug. Whenever a minecart, is on a powered rail that is not powered, when it gets a redstone signal the minecart just sits there. I really need this bug fixed. this effects, when you are gonna go on a roller coaster, and need to start going, and as you can see, the rail is powered, and the minecart is just sitting there.
Sincerely,
TheEpicKilljoy