Why datafile free space have but can't resize

I check my datafile has a lot of gaps but I can't do a reduce it. hope you help guide check the cause

SQL> select d.tablespace_name, d.file_id, d.file_name, d.bytes allocated, nvl(f.bytes,0) free

from (select tablespace_name, file_id, file_name, sum(bytes/1024/1024) bytes from dba_data_files

group by tablespace_name, file_id, file_name) d,

(select tablespace_name, file_id, sum(bytes/1024/1024) bytes from dba_free_space

group by tablespace_name, file_id) f

where d.tablespace_name in ('a')

and d.tablespace_name = f.tablespace_name(+)

and d.file_id = f.file_id(+)