Different TVs (and associated devices like your Freeview Plus recorder) have different 'sensitivities' to radio frequency signals. It seems that your recorder isn't as good at 'pulling in' weak signals as your TV is.
You
might need to upgrade your aerial system. (That means both your aerial and, just as importantly, the cable as well). If so, this aerial has been highly-recommended many times here on AB:
http://info.maplin.co.uk/module.aspx?moduleno= 22523
However, given that your TV can receive all channels (and your recorder can receive some), it would be well worth trying a cheap signal booster first. (Try Asda, Tesco Extra or Maplin Electronics. They don't cost much).
Chris