If it's all on the same aerial, then you must have some kind of splitter somewhere.
If it's a simple passive splitter, then each device will only get a fraction of the signal. A distribution amplifier (also known as a signal booster) might fix the problem.
Assuming you have a splitter then is it 1in 2out or 4 or 6 out? If it's more than 2 try just a 1in 2out version as that in itself will boost the signal.
Also try just connecting the aerial feed directly to 1 of the cables feeding 1 of the Youview boxes & see what signal strength you get then. Ideal to aim for is 60-70% , if it's down in the 40% range then maybe your aerial needs realigning or maybe it's not suitable for being in the loft.
Ideally, the booster would be installed at the point the aerial lead is currently split. If the aerial's in the loft, then it's probably also somewhere in the loft. Try tracing the wire coming out of the aerial.
It's better to use the booster as the splitter than to lose half the signal in the splitter, then try to recover it again with a booster next to the TV (or set-top box).