Gsp is known to like a constant flow a simple swivel of a power head could be the difference .
maybe it don't like the location were it sits to much light/to little
some times just blowing it off with a turkey baster getting any crud off of it works.
GSP is also a indicator that something isn't right in the tank maybe salinity to high
1.023 is ideal for it.
GSP is also good for not opening if you have a high ammonia level or high nitrate even high phosphates , GSP is sensitive to these , if you have any signs of hair algae or any other type they can cause false readings on phosphates.
checking some of the suggestions I mentioned should help you find the problem