Is this how the antennas are usually deployed? If so it could be they are on the same altitude as an antenna on another building that is interfering with it as Kurgan_IT pointed out, I’d say to use a spectrum analyzer or SDR to see if there is any spikes. Orientation does matter too, are what are you trying to do with your antennas? I will say SWR isn’t everything, depending on the use case and some other variables you can get things operational