Different devices (whether they be TVs, Freeview boxes, PVRs, or anything similar) have different 'sensitivities' [i.e. their ability to cope with low signal strengths]. Your downstairs TV set has lower sensitivity than the one upstairs.
The signal strength at your aerial isn't constant. It's affected by the 11-year solar cycle, individual solar flares, atmospheric pressure, weather conditions, interference from other electrical devices in the vicinity (such as your PC or your neighbour's TV set) and lots of other things. The signal strength used to be good enough for your downstairs TV to 'lock onto', but it no longer is (even though the upstairs TV, with its better sensitivity, is able to cope).
Start by checking that your aerial hasn't been knocked out of alignment by high winds. Then check that any plugs and sockets between your aerial are all making good connections. (It could be something as simple as he aerial not being properly plugged into the TV!). Then, assuming that you've not found anything wrong with the aerial or connections, buy a single outlet aerial amplifier from any large supermarket (such as Asda, where they're priced at £9.98, or Tesco).
If you've already got a signal booster installed (and it's not doing the job properly) you might need to get your aerial replaced by a better one.
Chris