we'd need more info to know, if it's a small fluctuation it's just normal daily changes in traffic, if it's going from $1 epc to like $5 epc and changing every day, then you might have a shave/scrub, or tech issues
Changes in your traffic
Changes in your ad position
Different days of the week
Different hour of the day
Not enough traffic to see consistent epcs
Random
Changes by advertiser (lp changes, shaving, backend changes)