FIrst thought is the symbol exploded or unexploded? If un-exploded then that is the problem. An unexploded symbol is one entitiy that is essentially locked. If exploded then there is more to look at. When you insert a symbol check the side menu to see if the "Explode" is toggled on. This is what you want if you plan on changing the properties of the symbol. You can also explode an already inserted symbol by using the 3D Explode function.
If the symbol was inserted with Explode toggled on and you cannot edit it you need to see if your Layer Search is on (if off you can only edit the active layer). Also see if any layers are locked.
Hope this helps