Recurring airlocks?

[plb]

My house is on three floor, and recently we started having a problem that has got more and more frequent - now it happens one or twice a day.
The bathroom at the top floor doesn't get any water - neither hot or cold, nor from the taps or the shower and the WC tank doesn't refill. The only way to fix it is to open the taps in the bathroom at the lower floor - then we get water immediately on the top floor. What could be causing this, and how can we stop it from happening?

 

[cider]

a post that deserves a sensible response !

guessing i would say you have low pressure which may be caused by an external below ground leak. You may have a check valve somewhere on your plumbing system and guess also you have a sealed system ( ie, combi) .
When you open a tap on the top floor there is a high resistance on the head of water required to reach the top floor through the check valve ( or to think of it even a loose jumper on a stop tap).
However when you open a tap on the ground floor the pressure is able to overcome the restriction of check valve or loose tap jumper and run as normal. The water will continue to flow and then reach the top floor until everything is shut off. Then the problem restriction will close and the fault will re occur.

 

[plb]

Thank you so much for your answer - I'm trying to digest it since it's a bit over my head.
I'm not sure it can be a pressure problem; we have a pump that kicks in, and I haven't noticed any drop in pressure.
We don't have a combi boiler - the hot water comes from a tank.
I don't know if it's relevant, but when I open a tap in the lower floor to get the upper one working, it takes a second to get the water; when the problem is not present, water comes out immediately.
Also, I have to regularly bleed the radiator in the bathroom at the top floor. Could it be that air is getting in the pipes on a regular basis, and if so what would cause that???? Sorry if it's a stupid question, I really don't understand much about plumbing but I'm trying to get some ideas since I haven't yet found a reliable plumber I can trust to come here and solve the problem. If I get some good hint from you guys, even if I can't fix it myself, I'll feel much better.