There do seem to be some locations that are particularly bad and for no apparent reason. One really bad spot for me is on and around the Millenium Bridge over the Thames. I have tried numerous times and can never get a lock there. I suspect either something is jamming it or there are reflections from the bridge structure. Standing in the middle of the Thames you are a long way from the buildings.

If you walk a few yards up to St.Paul's and sit on the steps you can then always get a lock (after the usual random waiting period of course) despite being surrounded by buildings and having a stonking great cathedral next to you!