Just curious as to what you mean by 'Poor Quality', Pixeling, Freezing, Grainyness or ???
MTV and the other Guru's can explain this expertly and I am NOT suggesting what I say is correct but you can have a transmitter running at full power but the signal it carries can be poor.
If it is affecting other people, then the problem is at the studio end but if its only you and the one TV, then its at your end and you will need to check everything out to find the problem.