I have used a SPOT extensively for the past 4 years when making intercontinental deliveries of new aircraft. There are "known gaps" in the coverage areas, but these gaps are well known and are usually in areas where motorcycles won't go, such as across the Bering Sea from Nome to Anadyr, or across the South China Sea. The gaps exist because the Globalstar satellite system (what SPOT uses) does not have universal coverage. Here is a coverage map:
SPOT Coverage.
I don't think there is a direct correlation between weather and SPOT connectivity. I've flown through some pretty bad weather (typhoons) and still had the SPOT track me OK. But, it is possible that heavy rain might attenuate the signals. An aircraft is normally above most of the rainfall.
Below are a couple of screen-shots showing
known gaps in the coverage. I would see these same gaps in the same places over and over again, month after month.
Michael
SPOT Gaps