There's a cap of ~60 debuffs on one mob anymore than that wont apply, half of those are visible (30). If your debuff is the 31st to apply it will go on but show no timer. If you reapply before the timer runs out, regardless of how many debuffs remain on the enemy, it will refresh the timer on the debuff in the hidden set so it will have full time again but not be visible. The only way for it to become visible again is to let it fall off completely and reapply it when the mob has <30 debuffs. This happens even if you have the option to only show debuffs you inflict turned on i believe, meaning you'd only see 2 dots on the mob then apply a 3rd which would apply and still only see the timers for 2 of them.