Problem with helicopter spawning in GTA 4, can anyone help?

Okay, When all 200 pigeons in liberty city have been killed, an annihilator spawns on top of a skyscr, and it can be reached through a window cleaning platform. If you have not killed all 200 pigeons, a maverick is supposed to spawn there instead. However, when I go to the helipad via the platform, the maverick simply will not spawn. It only spawns if i already have a helicopter, and fly over it, rendering it useless, as i already have a chopper. Does anyone know how to fix this or what i am doing wrong?