David,
I have a WS3600 and also get WD data from Heavyweather. I’m probably stating theobvious but here goes.
In control panel>station type make sure the La Crosse 3600 is selected. Confirm and set the location of history.dat in Heavyweather. Set History.dat logging internal to 1 minute. Make sure Heavyweather is also set to a 1 minute interval. DO NOT check “just use the fetch3600…”
In control panel>com port select a non existent com port like com 12. I don’t think this really matters, as I have never had a prob selecting com1 (where heavyweather is set) but I think I saw a post where someone did.
Try a 2nd install of WD (search the forum on how to do this) and set it up from scratch and see if it works. Edit: see this FAQ: Installing multiple instances of WD on your computer
Bill