Hi John, the other rads are further away from the boiler if that’s what you mean?
no, I mean the height (above ground level)
when there is insufficient water in an open-vented system, the water level drops, and the highest radiators in the house no longer have water all the way to the top. This will also reduce water pressure so that nothing comes out when you try to bleed them.
As mentioned above, this is most likely due to a blockage in the feed pipe that comes from the small tank in the loft, where it joins onto the mean heating pipes. This is often in the airing cupboard.
The risk of blockage can be reduced by using a system filter, which has a magnet to capture circulating iron oxide particles, and a corrosion-inhibiting chemical.
As you already have a blockage, it's too late to do that now, and a cleaning chemical will not work, though, once the blockage is removed, these methods may prevent a recurrence.
If you are fond of plumbing, you might be able to fix it, but more likely you will need a plumber. As its a water pipe, you don't need gas-safe qualification, though your heating engineer will be very familiar with it. I'd suggest that if you employ a professional to fix it, you ask them to fit a system filter as well. It may cost about £100 extra. You can add chemicals yourself.
Have you got a magnet? It will help you find the blockage.