No problem. I think it's a procedure to use if you somehow get air into the base unit's pump, which won't do it any good if it's on. It seems to just open the solenoid valves to let water flow under gravity, then pump a bit and repeat. I certainly did have air in my pipes and had to do this a few times to get it all out.
Thinking about your problem though, it could just be that you are running out of water. There may be several reasons or combinations... Here are my thoughts and a few tests to help narrow things down.
These power showers can pump out a LOT of water in a short time. It comes from your cold water tank and hot water tank. The hot water tank is supplied by the same cold water tank. So, if the mains water feed, through the ballcock into your cold water tank is slower than your shower is pumping it out, you will run out of water at some point.
My mains water pressure is pretty high but it's not like that across the country and it can vary from time to time. Neighbourhood usage can spike if everyone puts the kettle on during adverts on TV, dropping pressure, or more likely, having a shower before work. The water company may drop pressure for maintenance work. Any of this can mean your water inflow isn't as fast as normal.
Or, it could be something in your house. Your washing machine being on or someone else drawing water.
When you said "non-pumped" taps worked, remember that there's quite a bit of water in the pipes leading to the taps. That would have to be drained before you could tell if it's being replenished or not.
When my system had air in it, you could hear the motor speed up dramatically before the water stopped coming out. That may be a clue.
You could check how long the shower was on for when it seemed to run out. Clearly a long one uses more. I noticed in the manual that you should only run for a maximum of 15 minutes anyway, and not have another for 45 minutes, just to stop the pump from overheating. I must have a word with my daughter about that...
There's a flow rate adjustment on the remote. There's also a "Max Flow Rate enable" switch by the batteries on the remote. If this only happens 1% of the time, you could be just about balanced between mains water in and shower water out, with the occasional overstep. By using the presets on the remote with one lower flow setting, it might never happen again.
The last thing, after all else fails, I'd check your cold water tank and cistern in the loft to see if the valve or outflow pipe is working freely. I only say this because as a young lad, living away from home in a room in a country house, my housemate and I were surprised when water started pouring through the ceiling while we were watching TV. It was coming from the loft, so up I went. By torchlight, I found the problem floating in the cold water tank. A bat had fallen in, drowned, decomposed somewhat and blocked the overflow pipe. The cistern wasn't closing properly and it was floating, drawn to the outlet. If the cistern hadn't failed, the carcass would probably have sunk and been drawn over the outlet pipe.
Yuk. I thought it was just dead leaves at first.
And on that note, I bid you goodnight
Cheers
Vitch