Why does my cylinder keep losing pressure?

If the pressure gauge on your unvented cylinder keeps dropping back to zero within a few days of being repressurised, there is almost certainly an underlying fault. Repressurising it may temporarily restore the pressure, but it won't fix the reason the pressure is being lost.

If you're referring to the red expansion vessel, the first thing to establish is whether it is losing its air charge. The expansion vessel contains an air or nitrogen cushion that absorbs the expansion of the water as the cylinder heats up. If that charge is lost, the system pressure can rise significantly when the water heats and the expansion relief valve may discharge water through the tundish. Once the system cools, the pressure then falls again.

There are several possible causes, including:

The expansion vessel has lost its air charge.
The diaphragm inside the expansion vessel has failed.
The expansion relief valve is passing water.
The pressure-reducing valve is not regulating the incoming mains pressure correctly.
There is a leak somewhere in the system.
The pressure gauge itself is faulty.

A useful check is the tundish. When the cylinder is heating, look to see whether water is dripping or flowing through it. A continuous discharge could indicate that the expansion relief valve is opening because the pressure is becoming too high.

If the expansion vessel is repeatedly losing its charge, simply pumping it back up every few days isn't a solution. The system needs to be properly diagnosed to establish where the pressure is going.

Because this is an unvented cylinder, it contains stored hot water under mains pressure and has several safety devices. I would recommend having it checked by a G3-qualified engineer rather than repeatedly repressurising it yourself.

If the pressure is genuinely reaching zero, I'd get it investigated sooner rather than later rather than continuing to run the cylinder in that condition.