To add to James’ answer and save people time looking around, you could execute
CALL SYSPROC.ADMIN_CMD('REORG TABLE MY_TABLE_NAME')
via any available SQL client (i.e. even over ODBC or JDBC connection) to rectify this problem. However, the connection has to be in autocommit mode and you have to have admin privileges to execute this command.
I highly recommend to read the documentation on REORG before calling it.