You may have a corrupt setup file for that spark. Type these commands to a shell, replacing “LensFlare” with the spark name that is not working. (The setupch file only exists in some cases.)
% cd /usr/discreet/sparks/sapphire_10
% mv S_LensFlare.spark.setup bad.setup
% mv S_LensFlare.spark.setupch bad.setupch
Then try loading the spark again. If this fixes the problem, please e-mail us the bad setup files so we can inspect them.