Bank 1 sensor 1 slow response

I have a 1998 Mazda B4000, I have replace the o2 sensor that was suppose to be bad and still get a code reading. A couple of weeks ago I had a blown head gasket, cracked head and a new head replaced on the drivers side of the engine and the passenger side head worked, and a valve job done on my B4000.

After that the check engine light came on and I took my truck back to the mechanic and he read the code it said Bank 1 sensor 1 was "slow response". So I replaced the sensor on the drivers side and I still have the issue. What else could be wrong with my truck?