Do you have the TER logo on your site?  That's often the most common reason ads are pulled.

Also, are you limiting yourself to no more than one ad per seven days?

Also, have you gone to Account Manager and clicked on "my posts" to see if an explanation is given there?

If all else fails, write to Administration and ask them.

Ugh. I can't seem to be able to keep an ad  up for very long.
I just switched over to the new TER page and can't seem to find
the guidelines...
Someone help! I wanna post and be able to discuss on here!