DC Administrator account problem

Use my account lockout investigation process
https://www.experts-exchange.com/articles/29305/Active-Directory-Locked-Account-Investigation-Process.html 

Hi,
This may be happened due to replication issues If you have more than one DC.
Please check AD health and replication status if you have more than 1 DC.
Other reason is to cache old credentials in anywhere.

Simply  you can follow up the steps given in below references and try to isolate the reason.
https://social.technet.microsoft.com/wiki/contents/articles/23497.active-directory-troubleshooting-frequent-account-lockout.aspx 
https://community.spiceworks.com/how_to/48758-trace-the-source-of-a-bad-password-and-account-lockout-in-ad 

Try to filter out lockout Windows security event 4740 for admin account. Then analyze the time. You will be able to get any idea about this behavior.
Read event 4740 as below and try to identify the source.
https://docs.microsoft.com/en-us/windows/security/threat-protection/auditing/event-4740 

If the account is administrator, do you have your system exposed to the outside? By web, mail server, RDP?

Since you have a single DC, you can try and locate the source of the auth requests.
A clue as to what might be locking it is when it is locked, what issues are reported.?
Nirsoft if not mistaken has a lockout tool, but as noted single DC..
SHaun's siggestion or getting the aclock tool from MS that includes an eventmngmnt tool that will scour the events in the security log of the DC to identify the requests, failed, that lead to the lockout of the account.

How many people had access to the account.

Hello to all. THank you for all your answers and suggestions. We will look to this problem in the following 2 /3 days and keep you informed. Thank you again. Regard
Gad

thanks to all

thanks to all