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The problem happened just these days, it was normal before.
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Something has changed. If you are happy the agent set up has not changed, then the network set up has.

This is from metalink:

Here is what happens during the discovery process.
1. A hostname or ip address is entered from the EM Console.
2. This request is forwarded to the OMS.
3. The OMS takes that information and resolves it to an ip address (if needed).
4. Then in order to get the fully qualified hostname (which is needed
if dealing with multiple DNS domains), the OMS does a reverse NS
lookup and converts the ip address to the canonical hostname.

To simulate OMS discovery process, issue the following at a command prompt:

nslookup

The following information should be returned:
Server:
Address:

Name:
Address:
Aliases:

Next, take the ip address that was returned for the server and enter:

nslookup

The following information should be returned:
Server:
Address:

Name:
Address:

If the fully qualified name of the server is not returned, DNS needs to be
corrected so the reverse NS lookup name is correct or manually edit the
HOSTS files on each of OMS / Agent systems.
If the HOSTS file option is used, the entry should follow this format:



ie. 111.111.111.111 myhost myhost.us.oracle.com

This is with the fully qualified hostname, on port 1748, that the
Agent Gateway of the OMS contacts the Intelligent Agent to get
services information.