Attempting to manage a large number of selection sets simultaneously is not recommended. An AutoLISP application cannot have more than 128 selection sets open at once. (The limit may be lower on your system.) When the limit is reached, AutoCAD refuses to create more selection sets. Keep a minimum number of sets open at a time, and set unneeded selection sets to nil as soon as possible. If the maximum number of selection sets is reached, you must call the gc function to free unused memory before another ssget will work.