I live in Burkburnett Texas and my analog TV receives 4 over the air channels (KFDX 3, KAUZ 6, KSWO 7, and KJTL 18). However when I hook up my DTV converter box I can only receive KSWODT (7.1 7.2 and 7.3) which come from Lawton Oklahoma 40 miles away but the other stations are broadcast from less then 20 miles away. So why do I lose these channels?