I've had this issue before, do you use steam or Ea? If you use steam to run sim's then if you go the library in the steam application, there its a setting icon on the right that says manage. Click on that, then click properties, then another menu will pop up. When you see that menu click on installed files on the left side. The last option in the installed files is "Verify integrity of game files". Go ahead and click that, don't worry you don't lose anything, this option is just to make sure everything is working properly. If you don't use steam I'm not sure how to do this on Ea.
Another thing that helped me from having this issue, instead of "Save as", after you have already "saved as' just save from then on out. Sometimes when you save as when it's a save you've already saved previously, it can bug out and not save over the last save. I hope this helps you, I tried to explain to the best of my abilities.