Gears of war 2 horde problem?

i tryed to play horde from wave one, it shows the horde bar red as usual at the top left then reverts to the number 4 as soon as it starts and instead of the normal wave one its 2 boomers a wretch and a drone, and every wave after is all boomers. its very frustrating to die on the first or second wave i dont know why this glitch is happening but id appreciate some help figuring out how to fix it.