too many variables to answer really
is the aerial 100% correctly aligned?
is it a digital aerial?
is it a decent aerial or cheap one?
is the cable high quality?
are any plugs/sockets on the lead good quality and fitted well?
If you can answer yes to all of the above then you can try a signal booster, but if it's a poor signal they don't always work (your boosting the noise as well as the good part of the signal) or you can get your aerial up higher by putting in on a pole outside the house and not in the loft.