The problem might be a big table (many rows) but 0 Cardinality...
use dbname; show index in tablename; | Table... | Cardinality | | tablename| 0 |
Fix the table(s):
use dbname; ANALYZE TABLE tablename;
Alteratively:
mysqlcheck -a dbname show index in tablename; | Table... | Cardinality | | tablename| 12345 |