Hi,
I assume you are using the index table engine? The table structure has a 50 character limit on single words for performance reasons.
In this case, I would recommend switching back to the regular engine, or you can perhaps do an alter table query on the search database table, to allow longer words.
For that, you will have to access the database via a tool, like PHPMyAdmin or similar, and run the following queries:
ALTER TABLE wp_asp_index MODIFY COLUMN term VARCHAR(150) NOT NULL
..and
ALTER TABLE wp_asp_index MODIFY COLUMN term_reverse VARCHAR(150) NOT NULL
After the modification, you will have to re-create the index, so the data is populated again.