Check your router setup, make sure this check box is clear (No Tick) Advanced > Setup > WAN Setup > Disable IGMP Proxying
Freeview are frequently adding new channels, re-structuring and moving things around. It might be worth trying a re-tune of your box if channels are missing. Your box gives a good indication of signal quality of the channel you're watching in Settings > TV signal quality. Note that signal quality is more important than strength
You should have a lot more than 18 channels over aerial, you should have the same number as if you were watching with the aerial plugged straight into the TV. Have you tried re-tuning the Youview box?