If the circulation pump is operating, (you can tell by looking at the water feature on top of the filter box) and there are no bubbles coming out,then it could mean your non-return valve in the ozone line is blocked.
- Unplug the hose from the ozone box and see if it works.
- Disconnect the hose before the non-return valve and see if it is working.
- If it is still not working, make sure the circulation pump is operating and unscrew the top connection to the mazzei,you should see bubbles. (check where the blockage is, it could be in the hose itself and the hose would need replacing.