There can be alot of reasons for that happening. I'm guessing yourconflict is w/ settings & wifi use. If you have it set to use wireless netwoks to check for updates at specific times. If your using wifi at the time, Sense checks for the wireless network status to update. It doesn't always connect properly and that will often eff up weather sync. If you want auto updates,for best reults, you need to make sure it is set to use wireless network for the stock weather sync. If you Switching off to wifi nake sure you uncheck that box so it updates via wifi. effs up HTC sync. So go to settings >accounts, manually press the "Sync all" button and make sure the "auto-sync" button is checked. I'll be shocked if that doesn't solve the issue. Certain aspects of Sense are very finicky so keep tabs on any setting that involves network updating
P.S. Keep in mind a change in power saving settings or using 3rd party batt. saver apps can knock off HTC sync, as well.