My hams do this. I find that giving them fresh mix every three days only helps massively - they pick their favourite bits out on the first day, second day they eat the things they're less keen on and by the third day, all the least popular things have gone too. Obviously I give them enough at each serving to ensure they have enough to eat over the three days.
When I was giving fresh mix daily, I found I was throwing out a lot of the same stuff and once replenished, they'd just eat their favourite bits again. Now when it comes to time to feed on day three, there are just seed shells and maybe a couple of grass pellets left
