Some radiators are cold upstairs and downstairs

Hi
I live in a newly built house (3 years old) as is the central heating system. I have four radiators that are cold and all in different positions around the house, 2 upstairs and 2 downstairs, on both ends of the house.
I have tried bleeding the radiators concerned and this made no difference.
Has anyone got any idea what I should look at next? I have read most of these forums and do not have the hot at the top of the radiator and cold at the bottom, and also do not think a PowerFlush is required as the system is only three years old. If it helps the radiators are made by Myson and boiler by Ideal.
It is a very hard water area would this be causing a problem?
Please help.

Is the cold spot at the top or the bottom?

When you bleed the rads is the water black?

Does your system have a feed & expansion tank in the loft, or a filling loop and pressure gauge?

Has it always been like this or is it getting worse?

Have you got thermostatic rad valves?

What speed number is the pump set at?

Have you balanced the radiators?

Thank you for responding.

The whole radiator is cold.
The water was black.
Not sure the answer to "Does your system have a feed & expansion tank in the loft, or a filling loop and pressure gauge?" How do I find out?
It has got like this recently.
Thermostats are all on full.
How do I access the pump?
How do I balance the radiators?

So as you can see I know almost nothing about my system.

Have a look in the loft. See if there is a small water tank, about 18" x 12" x 12" with a large copper pipe curving over the top of it. That is what a Feed & Expansion Tank looks like. If not, have a look at your boiler. See if it has a pressure gauge on it (might be behind a flap) marked something like "BAR: 0 - 0.5 - 1.0 - 1.5 - 2.0" If there is such a gauge, see what the reading is. There would also be a filling loop, which is a flexible hose covered in shiny metal braid and having a control tap at one or both ends. It might be under the boiler or might be behind a flap. You should also have some boiler instructions.

If the water is black, that indicates there is corrosion and probably gas and sediment in the system. This is caused by not having a corrosion-inhibiting chemical added when it is new. You can get chemicals to clean the system and inhibit future corrosion. The sediment can clog the pump and cause blockages.

If you turn down the thermostatic valves on the radiators which are hot, then the others will probably warm up.

Have a look at the first 5 or 6 entries in this "Plumbing and Central Heating" forum.

Thanks again. I'll take a further look and let you know how I get on.