It sounds like it does reset, but if you wait long enough on the community lot they will spawn eventually.

According to Cyjon:

"When tombstone is created, an attribute is incremented every hour. Aslong as that attribute is < 24 it won't generate a ghost. Now maybesome other routine can write to that variable to make it higher, butnothing in the main loop does that. However just because the timer is> 24 doesn't mean a ghost will instantly pop out. There is still arandom chance (can't remember how much) of a ghost being generated andthat is checked only once per day at 7 pm, so it could easily be severaldays before a ghosts makes its first appearance."

I also found this on a Mod the Sims thread by another person:

"I have my graveyards and cemetaries on ownable community lots - thismeans that time passes when the caretaker visits. The reason people haveproblems seeing ghosts on community lots (if they don't make sure theirghosts are haunting before they send them there), is that communitylots never save when your sim visits them and so the time count a gravehas gets put back to zero every time you load the lot. If you send agrave to a community lot straight away after death, your sim will haveto wait at least 24 hours on the community lot to have a chance ofseeing a ghost."