It seems that in Elastic Search TEST is a reserved keyword

gistfile1.json

{"query":{"filtered":{"query":{"bool":{"must":[{"field":{"pool":1}}],"must_not":[{"field":{"user_id":"ed559296-41a8-4890-83c1-af1ef3959a83"}}]}},"filter":{"bool":{"must":[{"term":{"series_id":"TEST"}}]}}}}}

{"took":1,"timed_out":false,"_shards":{"total":5,"successful":5,"failed":0},"hits":{"total":2,"max_score":1.0,"hits":[{"_index":"ttt","_type":"series_subscriptions","_id":"ildella_bbb_series8","_score":1.0, "_source" : {"user_nickname":"ildella","user_id":"4a1f79e9-d231-455c-906b-fa650b9ca359","game":"bbb","series_id":"TEST","pool":1,"creation_date":"2013-01-10T22:15:06+00:00"}},{"_index":"ttt","_type":"series_subscriptions","_id":"abhishiv_bbb_series8","_score":1.0, "_source" : {"user_nickname":"abhishiv","user_id":"ed559296-41a8-4890-83c1-af1ef3959a83","game":"bbb","series_id":"TEST","pool":1,"creation_date":"2013-01-10T22:15:06+00:00"}}]}}

This first line is the query, while the second one is the result of
"curl -XGET 'http://localhost:9200/ttt/series_subscriptions/_search'"

Nothing gets matched!

If I change the "series_id" to something else, both in the query and in the data on elastic search, for example "test",
it works like expected and it returns the correct result.