One thought comes to mind here with your question for help.
Just how far are you from the repeater when your doing a test?
Is the other radio your trying to listen to close to the radio your trying to key the repeater with?
The reason I ask is if your any distance from the repeater, your probably swamping the receiver on the second radio you trying to use as a monitor. The closer the second radio being used to monitor the first radio, the more your going to swamp or de-sense the second radio receiver.
You may have other problems, but it always helps to point out a simple fact that most forget about.
Like has already been mentioned, the way you set up the squelch in the radio will also cause a problem. The use of the and / or selection, along with maybe the confusion of including MDC selection can cause major issues trying to receive signals. Take another look at this area and see if you might have over looked how a selection was set.