wiring a ballast from t12 to t8

i bought a ge ballast( gem220ts120). the t12 ballast had two reds,blues,yellows.I bought a t8 ballast with the same colors. I wired one blue from ballast to one blue from light tombstone,the same with the other blue.I did the same thing with the two reds,yellows color to color. I have two f15 lamps it is a two lamp fixture. when i turn on the light switch the lamps are dim. If i turn the lamps when they are in the light fixture they come on. And sometimes if i turn the light swich off,on quickly the light will come on.Could it be a bad ballast or wrong wiring.