Is the day of the week (and the time) reverting back several sim days as well? If so, then you are probably loading up the prior save each time and not the newer one(s).
There is a limit, it's difficult to define, on the number of saved games that the TS3 startup routine can display to choose from. If you have maybe more than 20, certainly more than 50, saves then possibly the newer ones just aren't showing up there. Can you delete some of the older saves from the TS3 user game folder in Documents (where your saves and added content are) if this is the case or move some of them out of the game folder for safekeeping if you wish to hold onto them all?