Radiator hot in the afternoon but not morning

[indus]

Hi
I've had an ongoing issue that the rads in my bedroom don't get hot. The bedroom is an extension and at the very rear of my property. The property is quite big at circa 4000sqft over three floors. However the ground floor is heated by a separate boiler.
Funnily enough the towel rails in the en suite get piping hot. All other room rads on the same floor get fully hot.

I've bled them (no air) and even had the plumber mess around with balancing but again no joy. The rads are about a year old and I have magnacleaners etc.

It's a system boiler and unvented cylinder. I've had Hive fitted about a month ago.
I've set the hot water to come on at midnight and stop at 0300hrs and then the heating comes on from 0400-0800hrs. I thought by not having HW and CH on at the same time this might help but these two rads remain lukewarm.

One thing I noticed a few weeks ago: If I turn on the heating for an hour or so in the afternoon the two rads then get properly hot, and that's with the hot water on at the same time!

Why would this be and does it give any clue as to what the problem might be?

Thanks

 

[Madrab]

Try running it in the morning with the hot water on and see if they get hot. If they do then someone's piped them from the wrong pipework and fed them from the primaries.