Hi

You could have some stuff in the pool that is using up the chlorine.

The key to discovering that is to know you free chlorine ( FC ) and total chlorine ( TC)
Subtracting them will give your combined chlorine ( CC )

If your CC is greater than about 0.5 then you have something in the pool that needs to be shocked out. And this could be why your levels seem to be running up and down a lot.

Can you test and get us those numbers ?? We could give you some better comments if we knew that.
When the pool starts to smell of chlorine it is also a sign of high CC. The smell, strangely goes away when you add more chlorine in a shock treatment.