If you're using a standard definition (SD) Freeview TV (or set-top box) you need to be aware that some SD broadcasters have to use HD 'multiplexes'. That means that you can't receive the signals via an SD set/box; you must have an HD one (even though you'll only be watching in SD).
In particular, 'COM7' is an HD multiplex but it also transmits the following SD stations:
Viva
CBS Reality+1
Rishtey Europe
Talking Pictures
5USA+1
So you'll need HD equipment to receive them.
http://www.radioandtelly.co.uk/freeviewchannels.html
If you've got other missing channels though it strongly suggests that your aerial is better at receiving some multiplexes than others, leaving you with some missing channels from the 'weaker' multiplexes. A simple signal booster (for around a tenner from most big supermarkets) should fix the problem. [You'll need to do a rescan after installing it].