For most domains like .COM or .WALES there are only a few situations where you're not able to transfer your domain name, these include if you have registered or transferred the domain in the last 60 days. Some registrars will also lock your domain for 60 days after changing the registrant contact details.
You may also wish to wait before transferring if your domain automatically renewed in the last 45 days as the time you paid for at your previous registrar may be lost.