I was getting an SES light on for code P1133 which is "HO2S Insufficient switching sensor 1/bank 1 fuel control shifted lean.

About 2 weeks ago I replaced my front 02 sensors with new GM parts. Reset battery now the code came back on today. This also happened last year the same code. I am starting to get annoyed and am thinking there is some other problem. Does anyone have any info on what could possibly be going wrong? or know a site that explains what is causing P1133 and what its definition means.
Thanks a lot