This is usually because something stacked in the machine is blocking the tablet from dropping down into the tablet collecting tray. This can also result in a tablet that is not fully dissolved or dishes that are not properly cleaned.

Make sure that nothing is blocking the dishwasher door or obstructing the tablet collecting tray.

You may also get only a partially dissolved tablet if the spray arm does not turn correctly. Check it is not damaged and that nothing is sticking down below the dishwasher basket and stopping it from turning.

If the tablet releases, but fails to dissolve completely, then there may be a problem with the water temperature of the wash cycle. Try a cycle at a higher temperature, but if this still does not cause the tablet to dissolve then there may be an issue with the water not heating correctly. You will need an repair engineer to look at the machine to establish the cause of the fault with the heating.