There are 3 reasons why AMP reports "SNMP Get Failed" error message:
1. AMP is able to ping the AP, but can't access the AP on port 161 (assuming that is the AP's SNMP port). Usually, if this is the case it's because a firewall is blocking port 161.
2. The community string that AMP is using to access the AP is wrong. You can verify the community string by going to the AP -> Manage page and clicking "View AP Secrets".
3. SNMP is not enabled on the AP.
Can you confirm that (A) you're definitely using the correct community string and that (B) the APs definitely have SNMP enabled? If you can, then I would be very suspicious of a firewall, an ACL or another networking problem.
Two troubleshooting techniques:
1. SNMP walk from AMP console:
There is an SNMP walk utility that you can use from AMP to test SNMP. If you can SSH into the AMP server, try this syntax for doing an SNMP walk:
sw <ip.address.of.ap> <community string>
As an example, I do an sw of an AP here by typing this
sw 10.51.1.207 public
If the community string is correct and the server is able to network to the AP on port 161, you'll see an output of the MIB; if not, you'll get an error (probably something like: "Timeout: No Response from 10.51.1.207").
2. SNMP walk using a third-party MIB browser. I use one from a company called MG-SOFT, and there is an evaluation version available here: http://www.mg-soft.si/mgMibBrowserPE-evaluate.html. It's fairly straightforward to use: enter an AP's IP address, click the hammer icon, verify the connection properties are correct, click OK. (I can help some if you have problems with this.) If it can get to the AP, you'll see data in the MIB tab; if not you'll see a TIMEOUT message in the lower panel. It would be interesting to try this from the network where AMP is installed and from the network where the APs are.