My index doesn't write anymore but read

Hi all,

We have a large database with postgre 9.3 (>300 Gb) and our queries are
(really) long for 6 days without changing anything.

After seeing our log, I saw that the same query on an old data was quick but
the same query with new data are really slow.
Let me show you a sample of my query on my table (>50 Gb):

Here, the query is really speed, less than 100 ms, thanks to the index on
date.

With this query, and the date after the 17th, the query is really slow (>10
000 ms)

How can we resolve that ?
A VACUUM FULL maybe ?
Why my index doesn't work for "new" data ?

Notice : we are working on SSD and the partition has space.

Thanks a lot for your help.

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