If you use the Manually manage setting, you'll have this problem. For example (worst cast), if your iPod had data corruption and you needed Restore (erase) it, you'll need to manually recreate all of those playlists on the iPod again, and that could a long time (even if you could do it from memory).
In this case, you'll need to manually recreate that missing playlist on the iPod. To do this, show your Music library in iTunes (click button for Music at left end of horizontal bar). Near the middle of horizontal bar, click Playlists. Along left side of window, the sidebar with playlists appears, with your iPod listed under Devices. Click small triangle to the left of the iPod (in sidebar) to drop down the iPod's content list. Click Music there, so that you are IN the iPod's content list, and you see the iPod's song list to the right. At the bottom of the sidebar, click the Plus sign and select New Playlist. The new playlist appears on the iPod's content list, where you can name it.
Does this playlist also exist in your iTunes library? If it does, select it in the sidebar under Playlists. Click any song to the right so that you are in the playlist, and do a Select All (Ctrl-A on keyboard OR Edit -> Select All from menu bar). Drag the selection to the sidebar and drop it on the new playlist in the iPod's content list. If the playlist does not existing in your iTunes library, click Music on the iPod's content list. From the iPod's song list, drag the songs you want to the new playlist on the iPod's content list.
That should solve your current problem, but going forward (long term), you should consider using automatic syncing to avoid this type of problem and make things more convenient, instead of the Manually manage setting. This will be easy, IF all of your playlists (and songs) that are on your iPods exist in your iTunes library. Select iPod in iTunes (click its device button on horizontal bar) to show its settings screen in iTunes window. Along left side of window (in the sidebar), under Settings, click Music. To the right, the iPod's Music settings screen is shown.
NOTE: Any songs on iPod that are not in your iTunes library (on the computer) will be lost.
Checkmark the box for Sync Music. Below that, choose the option to sync Selected playlists, artists, albums, and genres. Below that, under Playlists, select (checkmark) the playlists in your iTunes library that you want on the iPod. You can do this separately and differently for each iPod. If such playlists do not currently exist in your iTunes library, you'll need to create them first. Click Apply to sync your selection to the iPod. Going forward, update those playlists in your iTunes library. The iPod does not need to be connected. The next time you connect the iPod (or click Sync if already connected), iTunes automatically updates the iPod (both of your iPods) with the same changes.
If you ever need to do a Restore on your iPod (or if you need to get a new iPod), it will only take you a few minutes to set it up to automatically sync the same playlists from your iTunes library. Playlists will not get "accidentally deleted" on your iPod, because they existing your iTunes library.