Query_cache doesn't work with join

Query_cache doesn't work with join

I have a simple join query, but for some reason query_cache won't cache it!
SELECT id, news, approve, FIXED, DATE, allow_main FROM post LEFT JOIN
post_plus ON post.id = post_plus.news_id WHERE approve =1 AND allow_main
=1 ORDER BY FIXED DESC , DATE DESC LIMIT 7 , 7;
without the JOIN post_plus ON ( post.id = post_plus.news_id ) it's working.
query_cache is on
| query_cache_limit | 10485760 |
| query_cache_min_res_unit | 4096 |
| query_cache_size | 536870912 |
| query_cache_strip_comments | OFF |
| query_cache_type | ON |
| query_cache_wlock_invalidate | OFF |
Server version: 5.6.12-56 Percona Server (GPL), Release rc60.4, Revision 393