Some types of weeds die off a little at the end of the summer season, most likely resulting in decreased O2 levels. The fish may have moved out of this area if thats the case.
On the other hand, there are other weeds that thirve in winter, and attact fish. I have a similar spot that low quality weeds, and the fish are rarely there this time of year. Try setting up on the outsides of the beds and see if that helps.