You're getting the weaker commercial multiplexes, but missing out on the stronger ones. Translation: The problem isn't signal strength. You're getting the stations that are broadcast at the lower-power end of the scale.
You might think you have told us all that's relevant, but my guess is that there's a crucial bit of info missing. It could be that you have moved house, or that the TV was used some other place before, or you are hooking up to an old freeview TV or old freeview box, or that this is the first time you have been able to receive channels such as ITV2+1, or there's a join or kink in the cable somewhere. This isn't a complete list. But it does give you some idea of the range of issues that might be affecting your reception in addition to anything directly related to the aerial set up.
You have already done some basic checks. The question about aerial orientation is easy to answer. Go look at your neighbours aerials. You are looking for two things.... the first is direction. Are they all pointing in the same direction? This tells you if they are all aimed at one transmitter. If they are, then make sure yours is pointing in the same direction too.
The second is the orientation of the aerials. Are the blades on the long boom lying flat or pointing up so they look like a fence? I suspect your neighbours aerials are pointing at one of the two main transmitters for the Belfast area. Aerial orientation for main transmitters is usually horizontal rather than vertical.
If you have done the above then I think it could be that you need to do a factory reset or First Time Installation or whatever the equivalent "start from scratch" mode is for your TV. This will clear down the channel memories and replace them with fresh. So, as long as your TV is capable of receiving the current standard of Freeview transmissions that's about as much as you can do with what you have at this stage.
There is still the possibility of signal overload or the TV/Freeview box being older versions that can't cope with the newer version of Freeview transmissions. There's also cable issues.
If you have too much signal then you'll see this on the signal strength meter if your TV. It'll be close to- or totally maxed out. If it's like that on the lower power stations (ITV2+1 etc) then it will definitely be overloading on the full power stations such as BBC1. The solution is an attenuator. It's something that reduces the signal level.
see this link You can get ones that look like an inline adapter that drop the signal by a specific amount depending on the value, or ones that are variable. If you are troubleshooting then the variable one is probably easier.
Having a non-compatible Freeview box then the simple answer is just to buy a new one.
Cable issues: I have had it where a customer did a DIY install and bent the cable sharp round a corner. It acts like a notch filter selectively removing a frequency. Over all though, my money is on a factory reset/retune. Try that first.
