If they're not eating them all it is usually an indication of either:
- diet too rich or new things added too quickly
- an upset to the normal gut system, e.g. because of antibiotics
- a 'change in flavour' making the bun not like the taste, e.g. some medications can cause this
- dental problems, meaning the bun is finding it hard to reach round and get them
- spinal/arthritis or weight problems, meaning the bun is finding it hard to reach
So really you need to consider all the factors to determine what might be causing it. What do you normally feed?