Cold Radiator

There is 1 radiator in my house which has always been cooler than all of the others in the house. Having just installed a new boiler i decided to have a look at what the problem was with this radiator. It looks like the problem is with the thermostatic valve as the pipe up to the valve is hot, but the radiator itself never gets close to the heat of the pipes feeding it, or the other radiators in the house. I took the valve off to have a look at it and refitted as the thermostatic valve i had bought to replace didnt appear to fit as the connection between the rad and the valve was a different size! The old thermostatic valve was put back on and the system was refilled, during refilling i bled the radiator. Once the system had heated up all radiators bar this one were hot. What i dont understand now is that the 2 pipes connecting to the radiator are stone cold, yet all other radiators in the house are fine. Is the problem the valve that i took off? Thank you in advance for any help that you may give.
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Is this a steam system?
If it is steam, maybe it's a bad steam trap.

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system the bleed valves on the radiator were leaking water.
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