Materialized View Refresh Error

Hi
I am trying to load bulk of data(millions of data) using dat files with Oracle 10g . I am doing this to get details about how much time it takes to refresh the materialized view when loading the table.  When I was loading the data for the first time, it refreshed appropriately for the first 3 batches of file and then the state went to stale. So, I tried to refresh it manually using dbms_snapshot.refresh('mv_table','f') basically Fast Refresh. When i executed it, it gives me the following error:

EXEC DBMS_SNAPSHOT.REFRESH('mv_lrx_script_3','f')
ERROR AT LINE 1:
ORA-12008: error in materialized view refresh path
ORA-01426: numeric overflow
ORA-06512: at "SYS.DBMS_SNAPSHOT",line 2255
ORA-06512: at "SYS.DBMS_SNAPSHOT",line 2461
ORA-06512: at "SYS.DBMS_SNAPSHOT",line 2430

But when I executed dbms_snapshot.refresh('mv_table','c') (Complete refresh) the procedure is completed successfully without any error.  

My questions are:
1. What does the error means and what's the solution to this?
2. Why is complete manual refresh not failed?Does that mean the query (select count(*) cnt, usc_cd, cmf_prod_nbr, cmf_pack_nbr from lrx_script_3 group by usc_cd, cmf_prod_nbr, cmf_pack_nbr) which is used to create columns in mv table doesnt meet the requirement of fast refresh? If so, how does it refreshed for the first 3 dat files..
3. Why is it the MV refreshed for the first 3 dat files and failed to refresh only for the 4th one?

Can anyone throw light on this?

For your understanding, I also attached the creation of materialized view and view log..

Thanks in advance
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create materialized view log on lrx_script_3
with rowid(usc_cd, cmf_prod_nbr, cmf_pack_nbr) including new values;
 
drop materialized view     mv_lrx_script_3;
CREATE MATERIALIZED VIEW mv_lrx_script_3
  TABLESPACE TS_LRX_DATA04 NOLOGGING
   STORAGE (INITIAL 512K NEXT 256K)
   BUILD IMMEDIATE
   REFRESH FAST ON COMMIT
   ENABLE QUERY REWRITE
AS
select count(*) cnt, usc_cd, cmf_prod_nbr, cmf_pack_nbr
from lrx_script_3
group by usc_cd, cmf_prod_nbr, cmf_pack_nbr