The AGCH Success Rate must be about 99%-100%.
You could rectify the problem. If the problem is on AGCH Blocking, you could add AGCH subslot, but remember the AGCH subslot is shared with Paging Subslot in A one CCCH. Adding the AGCH subslot will reduce Paging capacity.
You must monitor first the paging capacity. If AGCH and Paging are high utilization, so you must add new site or sharing with other cell