distribution--cleanup agent not working properly

hi all we have a situation in our office.the distribution clean up agent is not getting rid of the old transaction.what could be the reason do you gurus faced this problem before.what are the effective ways of shrinking the distribution databaseplease share your expereiences.thanks,lara

Do you see any error message in error log or job history?
Is replication cleanup job running?

i dont see any error messages and yes the distribution cleandup is executing fine every 4 hrs.thanks,