Point 1:
Different makes and models of devices with TV tuners in them (such as TVs, Freeview boxes, PVRs, etc) have different 'sensitivities', which means their ability to handle weak signals. varies
Point 2:
TV broadcast engineers have always worked to standards which should provide a good signal to anyone with a high quality EXTERNAL aerial. While some people have managed to get reasonable reception by using a set-top aerial (particularly if it's got a built-in booster like the one you seem to be referring to), it has never been the aim to make such reception possible.
Point 3:
When the switchover to digital TV was first announced it was expected that up to one third of viewers would need to upgrade their aerial installations. While the actual number has turned out to be far lower, it's hardly surprising when some people experience problems with signal quality.
Point 4:
In areas where the old analogue transmitters have yet to be switched off, all Freeview transmitters are currently operating on reduced power. (That's necessary in order to avoid interference to the analogue signals).
You're not going to get reliable reception on your new TV until you find a way to connect it to a decent EXTERNAL aerial. You can probably use your existing aerial (as long as it gives good reception with your lounge TV) but you'll need to split the cable as close to the aerial as possible, perhaps with the addition of a mast-head amplifier. It's time to call in a reliable aerial fitter.
Chris