Your question is kind of like "my car is supposed to have an engine, now it doesn't start. Whats wrong?"

In order to give you the best help, we really need to know what brand an model it is. A picture would also help.
However Thenasty is correct, if you can swap from 8 to 4-way there should be some kind restrictor plate controlled by some kind of actuator.
On some models it is fixed with some screws that has to be removed etc.
In your case it might be the situation that the 4-way restrictor is removed completely and then you can move in 8 directions.