Get's back to we are volunteers, we have a life, we have a job, we need to earn money, if we don't work we get no money and then life becomes hard. Some people are not around at certain times of the year due to work commitments (just because my car is in the driveway does not mean I was to slack to turn out, I might have worked the last 12 hours on afternoon/evening shift and now I am getting some important sleep to be able to do it again tomorrow so I have my pager turned off) There are also some people that will only ever be around during the summer as they are interested in helping the community fighting bush fires but are not wanting to get caught up in the rest of the stuff. Well that is there choice and boy are they handy over the summer, they don't need to be penalised because they don't want to go to VA's etc.
Too many people are interested in getting rid of so called dead wood without looking at reasons as to why they might not be around at certain times of the year and taking into account all the extra work that they have done over previous years.
How to get rid of a volunteer, never acknowledge the extra work that they might put in (whether it be vehicle maintenance, radio checks, cleaning equipment etc etc) and just have it that you expect them to continue to do that without any help or thanks and the one time they are away, working or unwell jump all over them for not doing it