Co-BCCH_NSN (Need solution onTAFR prb)

i’ve got a situation. after implementation CO-BCCH Concell,Dualband 900/1800)in NSN network, TCH assignment fail radio(TAFR)went high.we have more interference in 900 & less in 1800.previously BCCH was both in 900 & 1800. after implementation it is only in 900. In that case TAFR may increase a little bit.It was managed to low down TAFR by several parameter tunings, But my quary is when we uptilted the 1800 antenna, TAFR went more high whcih is unusual. caz this bandgap minimization should resolve the prob, but actually it didnt. can anyone explain what could be the root cause behind this….?