distribution cleanup jobs cannot work

hi ,

i have one question about replication. it is sql server 2008 R2+sp3.

we have built a huge replication architecture, i found the size of one instance's distribution database grown to 70 GB.

it shows that the distribution clean up job doesnot work properly, and the job runs normally, but removes 0 records all the time.

it should have some orphaned data in msrepl_commands or msrepl_transactions.

my question is:

how can I find out the orphaned records, and then I can remove them,at last the distribution cleanup job can run normally.

and another question is : ;how did the issue come out?

thanks