Cisco 2900 Series Routers Unable to Consistently Resolve NHRP Addresses

I have implemented two different Cisco routers at a site that have both run into the same issue. For a handful of spokes in the DMVPN "cloud" they cannot seem to automatically resolve the public IP address mapping for the private address automatically. I am having to manually go in to the tunnel interface and statically define the mapping. What is strange is that it is successfully mapping the addresses for around 80% of the locations and it isn't 100% consistent with which locations it is struggling. When this problem is occurring for a given location I will generally see an entry under sho dmvpn of:

​

# Ent Peer NBMA Addr Peer Tunnel Add State UpDn Tm Attrb

----- --------------- --------------- ----- -------- -----

4 UNKNOWN 192.168.1.10 IKE never IX

0 UNKNOWN 192.168.1.50 IKE never IX

0 UNKNOWN 192.168.2.10 IKE never IX

0 UNKNOWN 192.168.3.10 IKE never IX

​

Additionally, it has these entries for individual hosts on the subnet behind the DMVPN router that provides connectivity to the site. The workaround I have been doing to statically resolve for a site is under the tunnel interface:

​

tunnel 1

ip nhrp map 192.168.251.1 xx.xx.xx.xx (xx's represent the public IP address)

​

After completing this manual mapping, connectivity to the site works without issue. The primary symptom that brought this to our attention was users getting one-way audio between sites, because interestingly enough, the other side of the spoke does not have this issue (other than the user at the problem site complaining about one-way audio).

​

Any help or advice on this issue would be greatly appreciated.