A couple factors can contribute to changes in the way a stove operates. I assume (and you know what that gets you and me!
) that the timer settings and other controls have not changed. If you short across the Tstat terminals and make the stove run constantly does the fire get bigger? if not you may have a bad relay.
Typically if the stove requires max feed to get a decent fire then you are restricted on either the amount of coal being pushed or the amount of air to burn the coal. as you suggested the cam could be worn out, especially after 12 years, so a thorough inspection would be the first thing to perform. Then there are a few other things to consider, such as:
One item that can choke off a fire is the small pieces of coal or "fines" as they are called that fall into the plenum under the grates. This can clog the air flow which may contribute to having to supply more coal to get the same size fire, this would also lead to some unburned coal in the ash pan. Have you ever cleaned that area out?
Another issue affecting burn can be your coal supply, has your coal supplier changed or maybe he is getting coal from a different source?
You said you replace the vent fan. do you mean the combustion fan or the fan that blows hot air out of your stove (convection fan)?
The main thing is don't give up, your stove is having a problem and you will definitely be abel to figure it out with a little patience.