O2 sensor question

I have a 1996 silverado k1500 5.0 litre extended cab automatic.148888 miles with a 02 problem. One year ago the check engine light came on and I have a OBD2 reader and it said O2 sensor low circuit voltage.I replaced all of them myself and I had the catalitic converter changed because it was bad.8 months went by and the check engine light came on again but before it came on the truck started running bad like it was on 7 cylinders and then about 5 minutes later the light came on.I reset it and it did not come on untill about a month later and I did the same thing again.I took it to the chevy dealer and they said it was the o2 sensor bad so I replaced the one they said was bad myself about three weeks ago. Now the light came on again and it said this time o2 sensor ckt no activity bank 1 sensor 1 why is this happening so much. The dealer told mr that the truck checked out ok and that was the only thing wrong was the sensor.