That's because they're cack.
Most I know inc myself get the right one falling out (not Ipod).
I use In-Canal ones now, far better.
I saw Sennheiser CX400s recommended in Custom PC mag, had a look online and found them for £10 on that famous auction site! Absolute bargain, they probably fell of the back of a lorry but I'm not complaining.
Do yourself a favour and get some canalphones...better sound quality, less overflow, more comfortable.