If you make maps or objects or download objects sometimes this happens.
If so you go into Object Maker and open one of the object groups, then unfortunately you must go through each object and save it into a new group. IE. All your rocks are missing so you would go to MyRocks then load Small Rock. Instead of saving it to MyRocks though, you would save to MyRocks2 or something.
But yeah re downloading it should fix as well xD