The cotton gets more dense. This then acts as a particle filter that strips away the flavour. At least this is how it appears to me.The cotton also seems to lose it density maybe?
You will notice you start to get dry hits even when there is juice in the cotton still. This is a clear indication that it is too dense to act as a true wick.
I'm 100% with Mathee. If you run cotton, change it daily!
Silica can go a couple of days without changing.