It would help to know 3 things:
1. version of Moodle
2. how are you hosted? Shared?
3. how often are you running your cron job? (related to #2)
Not all of the task in the cron job runs at the same time ... there are task contained in the cron that could be heavy processing. When Moodle hits one of those and can't finish for some reason, an entry is made in adhoc_task tables and the next run of whatever task tries to pick up where it left off. That clear as mud? :|
IF you followed directions for setting up the cron job it's redirected to la-la-land (doesn't keep a log of it's own). Therefore, no easy way to see what is failing.
So, look in code directories for error_log files. They could be in any directory. Those are php errors ... and could give hints/clues as to what is failing and why.
Are you running anything 'process' heavy? ... like automated backups?
Might run this query on your DB for Moodle:
select component,nextruntime from mdl_task_adhoc;
Should give a clue ...