Missing Arrows on Outlook Reply or Forward Messages
They say if you can't figure out what's causing it, replicating the problem may be the next best thing. There is no exact science but here's what I found and you may also want to check it. I believe this is why the arrows randomly show (or don't show):
Your desktop or workstation's time/clock is not exactly the same as your Outlook Exchange time. They are at least a few seconds different. If you replied or forwarded an email immediately after receiving it, and your workstation's time is off by even a minute against the message delivery time, NO ARROWS will show. If you want to experiment, you may change your desktop time by at least a minute so the difference is a little more pronounced. But please note that when you change your workstation time, you have to log-in again (restart). Some network security apps prevent workstation to authenticate on Active Directory if the difference in time is more than 5 minutes so be sure to change the time difference under 5 minutes.
There is a Microsofy KB on this issue but there's no mention on the time difference as the cause of the missing arrows.
To my organization, it's not a big deal since Sent Items are tagged anyway but I happen to notice the arrows on my own.
Hope this helps. I know it's an age old question but at least it can relieve some of the head-scratching.
Cheers!