By default, all hard disk drives / SSDs should have associated tray icons IF they provide temperature value.
On Configuration -> Hard Disk Drives page, it is possible to configure which particular hard disk drives / SSDs should have tray icons at all: by DOUBLE-CLICK you can toggle icons for any particular disk drives.
> Not sure if that's what's causing it.
I'm also not sure. Without knowing which version you used previously, hard to say anything for sure.
Generally there is no change between the previous 5.61 and the newest 5.70 in terms of tray icons, so if you used 5.61 version, then things should be 100% same.
> I checked my hard disk preferences and one of them was not selected to show. But even after selecting all of them, only 6 out of 8 are showing.
Interesting. I'd try to toggle on/off each of them and verify what happens: if your Windows may show only 6 icons (regardless of which ones selected) or the icon of some particular disks can't be displayed at all.
Generally the tray icons are displayed by Windows itself. It is possible that it may hide the icons by default, so you may need to check the tray icon settings of your system too.
If you use Report menu -> Send test report to developer option, it may help and give some thoughts.