Pruning not occurs for a simple query involving subpartition table on 10.2.0.5 ...

Hello I am new in the community. For advance, excuse  for my level at english !!
I search for a reason why dynamic pruning doesn t occur for my query

I have a subpartition table :
- range partition on month pk_id_an_mois  (example : 201005) => 1 partition by month (from 200401 to 201501)
- hash subpartition (128) for id_agence pk _id_ag (code of agency => about 115 distinct values)

adm_user is a parameter table which lies a login with an agency

for the following query

SELECT  adm.adm_users.nom ,
         SUM (  NVL (dtm.dtm_agg_agvte_prd.mt_ca_c_a, 0)
              + NVL (dtm.dtm_agg_agvte_prd.mt_eco_cntb_c_a, 0)