Have two Forests with trust between. Suddenly trust is not working from Domain in Forest A to Domain in Forest B, but still working the other way around.

This happened once before due to time being out of sync, at which point I configured Forest B to retrieve time from Forest A and no issues since and definitely still in sync now.

Checked all the trust configuration and everything validates both ends OK. Either end can resolve the other domain and such fine.

Here is the error logged on the DC over in Forest B when someone from Forest A attempts say SMB access:

Log Name:      Security
Source:        Microsoft-Windows-Security-Auditing
Date:          1/09/2015 12:35:25 p.m.
Event ID:      4625
Task Category: Logon
Level:         Information
Keywords:      Audit Failure
User:          N/A
Computer:      DC01.DOMAIN-B.COM
Description:
An account failed to log on.

Subject:
Security ID: NULL SID
Account Name: -
Account Domain: -
Logon ID: 0x0

Logon Type: 3

Account For Which Logon Failed:
Security ID: NULL SID
Account Name: username
Account Domain: DOMAIN-A

Failure Information:
Failure Reason: An Error occured during Logon.
Status: 0xc000005e
Sub Status: 0x0

Process Information:
Caller Process ID: 0x0
Caller Process Name: -

Network Information:
Workstation Name: Domain-A PC
Source Network Address: 192.168.X.X
Source Port: 54282

Detailed Authentication Information:
Logon Process: NtLmSsp 
Authentication Package: NTLM
Transited Services: -
Package Name (NTLM only): -
Key Length: 0

This event is generated when a logon request fails. It is generated on the computer where access was attempted.

The Subject fields indicate the account on the local system which requested the logon. This is most commonly a service such as the Server service, or a local process such as Winlogon.exe or Services.exe.

The Logon Type field indicates the kind of logon that was requested. The most common types are 2 (interactive) and 3 (network).

The Process Information fields indicate which account and process on the system requested the logon.

The Network Information fields indicate where a remote logon request originated. Workstation name is not always available and may be left blank in some cases.

The authentication information fields provide detailed information about this specific logon request.
- Transited services indicate which intermediate services have participated in this logon request.
- Package name indicates which sub-protocol was used among the NTLM protocols.
- Key length indicates the length of the generated session key. This will be 0 if no session key was requested.