The satellites that process our high-definition sea surface temperature (SST) and chlorophyll data can't "see" the ocean surface through clouds, smoke, fog, or intense sun glare. They need clear skies in order to read the ocean surface and collect temperature and water color information so that we can process it into our high-def images.
A good way to check is by looking at the True Color Satellite Imagery layer, which shows where and how extensive the cloud cover is over the ocean that day.
In some regions, especially in lower latitudes closer to the equator, a few satellites don't quite cover the entire globe when they pass overhead. Those data gaps will also show up as blank spots on the map and are easy to spot as sharp straight diagonal lines where the data ends.
There may also be areas where the glare from the sun is just too strong and it corrupts the data - we filter all that out for you. But that's why we collect data from multiple satellites every day, to make sure we still get full coverage of your region even when one satellite misses a section.