I think this subject might have been addressed in a previous post. Here are my suggestions. Figure out why so many sisters are missing events. Is it senior apathy? Is it the time that the events are scheduled? Maybe you can change when the events are held to determine when the most sisters are available to attend. Since fining isn't a deterent, maybe now you can have an amnesty or reimbursement program. If the sisters who have been previously fined attend two or three events, you can reimburse them for one missed event until they are even. Hopefully your chapter hasn't allocated those funds for anything.
My chapter didn't have a color classification system that UMgirl suggested (btw that is a great idea!), we did classify certain events as mandatory or optional. Each sister needed to attend a minimum number of events in order to remain in good standing and attend the formal.
Good luck! Unfortunately, this seems to be a problem that plagues many chapters.