Desk pens tend to need sac replacement more often than pens kept either nib-up or horizontal. The hanging load of ink tends to create a partial vacuum inside the sac, which, in time, will flatten it. The flat sac takes a set and will not suck up a load of ink when a refill is wanted. I have a Touchdown desk pen like that now. Luckily, Touchdown pens are the easiest pens on the planet to re-sac.