Chunks unloading and staying unloaded

On the multiplayer server i play on chucks sometimes unload and stay unloaded. I f3+a and it won't load, we restart the server and it stays unloaded, and we just reload the server and it won't reload. All of the players on the server see these unloaded chunks. I will somtimes appear in places where we have built things and it bothers us losing what we built. Is there any solution so far? Thank you.