In my case with the METAR, I'm pulling from NOAA and I know this is a VWS problem. It only happens on two specific METARs, KFMY, and KSRQ. All others (about 5) that I'm pulling work just fine all the time. It doesn't matter where those two fall in the order either. Monitoring it with a protocol analyzer I can see that VWS doesn't even attempt to pull the data for the METAR. However, when it's done pulling all the other METARs it writes a blank METAR file for the two I have an issue with. If I delete the METAR, then add it back in, and restart VWS, it starts working properly for a few days before it starts doing it again.