I work for a TO in resort - not FC, and as part of my job I answer complaint letters sent from UK. (I don't deal with compensation offers, that is done back in the UK.)
The majority of complaints I deal with are not brought to the attention of the rep while in resort but we still have an obligation to answer them.
It does weaken the case somewhat if complaints are not reported at the time as has previously been said, it is deemed that if you couldn't be bothered while you were there then the problem was not that bad.
Even if a property is not repped every day, there is always a reps board or folder containing local office and out of hours contact numbers. Also, there is usually a phone number on the booking itinerary and failing that a guest can contact the UK office. So there is no reason not to make a report while on holiday. Even if the rep can't fix the issues, at least the complaint is recorded.
Re compensation, it depends on the issues and how far you are willing to take it.
karenl, can you tell us what happened?
Louby